Further experimental data: wiped the seat with straight bleach. Did not remove any stain, and very little blue came off on the wipie. Less than came off with alcohol.
Okay, next project: replace toilet seat before my book club meets here after Christmas. I don’t want to have this conversation with 10 other women, and the seat looks ridiculous with blue butt-shaped stain.
Anything I should be aware of when buying a toilet seat? IOW is there any way I can screw that up?
I would try a MR CLEAN magic eraser first. You may want to consider a slow release lid, no sound rather pleasant, TOTO makes them.
Immaculate conception - get ready for Jesus! :eek: 
“Foretold it shall be by thy holy blue toilet seat . . .”
Back to give you my theory. There is a blue dye that only dissolves in sweat but not in water, and only in certain people’s sweat… I read about it in a book of medical curiosities once then have seen it or heard of it a few more times since. Usually what happens is that a patient comes in worried about progressively turning bluer/cyanotic. Showering/washing does not remove the dye but it does come off with alcohol. I have tracked it down to new sheets and a new shirt in patients. Usually the offending garment is bright blue but the stain is a bluish-gray. The patient will have washed the item and showered but the dye only comes off with alcohol. Perhaps there is something the OP has bought recently that is bright blue (the offending item is usually bright blue, not jeans) and the OP is only mildly reactive so that the dye is not enough on the skin to notice but does come off when sweat transfers to the seat. It may be that the hormonal changes in pregnancy make their sweat more likely to dissolve this dye. In any case, the key is that the color comes off the skin with alcohol, leaving the wipe bright blue. See if there is anything you have used recently (clothing, towels, sheets) that is bright blue.
If I’ve said it once, I’ve said it a thousand times: that’s not what the Immaculate Conception refers to.
Buy a blue toilet seat.
I think I remember this happening to me once, a few years back. It was one of those padded toilet seats. My memory of the incident is a bit hazy, but I think at the time my theory was that it was caused by my cleaning the seat with some sort of household cleanser - probably lemon scented “All Purpose Cleaner”. The stuff wasn’t blue, but I presumed there had been some sort of chemical reaction with something in the plastic. Anyway, it resulted in quite pleasant shade of pale blue, and the coloration was a bit more even than that shown in the OP’s photo, so I didn’t worry that much.
I told you I was speshul.
Yes, people, it’s the Virgin Birth.
I thought about that. In fact that makes sense, because maybe this will happen again.
It’s not the color per se; it’s the fact that it’s so clearly a butt print.
It’s a blue moon.
njtt’s experience is consistent with my proposed hypothesis. Using an acidic or abrasive compound all over the seat will make the whole seat susceptible to staining whereas acidic sweat slowly eroding will make only the area of regular skin contact so prone. Once prone any compound with blue dye in it that gets in contact, such as but not restricted to aerosolized blue in toilet water, will cause the blue staining. This also fits as being more common with pregnant women (heavier and spending more time on the toilet more often).
Of interest blue dye is in lots of things that do not appear blue. Black pen ink, for example, is mostly a very concentrated blue dye with a few other agents added as well.
Need a bit more than this.
What is special about certain people’s sweat that only their sweat, and not water and not other people’s sweat, dissolves only this special blue dye? And why complete butt print?
I do not doubt you read it somewhere but it sounds like something that someone was creating as a truth rather than being something real.
Just a guess … Heat. Your butt cheeks warm up the seat and the blue dye from the spume from flushing attaches to the seat.
Toilets and their seats come in two varieties - call them regular and long. If you mix them it looks weird and the seat may not be level.
Padded seats seem like a good idea but are often flimsier - what sort of - errr - stress will it be subjected to?
Also, the low end (Pardon the pun) seats tend to have flimsier nylon hinges that allow the seat to move around more and can lead to a nasty pinch :eek: if things align just right - more so for men, but another post seems to indicate this isn’t a concern on a regular basis.
If the one you are installing has metal bolts, be careful about over-tightening; you can break the porcelain body of the toilet. :smack: Just tighten until the seat won’t move around. I think most use the non-corroding nylon bolts these days, which lessens or eliminates the chance of this happening.
Other than these caveats, it’s real simple to change the seat; the modern hardware usually only takes a large screwdriver.
Of possible relevance:
The nitrogen compounds in urine will facilitate dye transfer; look up the traditional dying process for tweed cloth.
Natural indigo dye seems to be particularly susceptible.
TMI ALERT :dubious:
A few times I have ended up with a blue dot on tighty-whities from a stray drop.
Is there any chance that was originally a blue toilet seat and someone painted it white and for some reason the paint rubbed off?
Maybe the seat achieved sentience and was embarrassed to see a derrierieriere coming at it! :eek:
Yes, I think mine did show some evidence of the blue being butt-print shaped, but not so blatantly so as yours (perhaps because mine was a padded seat that squished under my butt). I wonder if the cause could be some sort of three way interaction between the plastic, household cleaner, and some chemical in sweat, or skin oils. I am pretty sure mine did not have anything to do with dye from denim. I was not much of a jeans wearer at the relevant period of my life.
We had that tile in my parent’s (small) bathroom in the house I grew up in (in White plains, NY).
Aniline dyes are amines and can be blue. Urine and sweat are both nitrogenous.
It sounds like something that no-one has been curious or well enough funded to do definitive research on, so we may have to accept theory.
ETA:
And I love that kitchen.
I will be having house tours shortly. 
(It IS the cutest little craftsman house you’ve ever seen.)
The plot thickens.
The blue toilet tab has been removed from the investigation as a tab of interest and the new/cheap blue jeans are back in the dock.
This morning I took my sheets off the bed to wash them and there is a big blue area in the zone where my body comes in contact. It’s mostly on the bottom sheet, but some on the top sheet, too. Also, I sleep with a full-length body pillow and often put one leg over it (helps support the back) and that pillowcase was blotchy with blue, too. All items in the wash now. I’ll be curious to see if the dye comes out.
The culprit is clearly the jeans I got a short time ago from…you guessed it… Wal-Mart.
This is my theory: Wal-Mart has extra dye put into its jeans where they’re made in the sweatshops (aptly named) of the Third World. That way, when The Big One happens, Wal-Mart will be able to identify its customers by means of special lights that will detect on their skin the indelible color (which neither the naked human eye nor the eyes of naked humans cannot) and rescue us from the Apocalypse. Of course, they’re not really rescuing us–they’re saving us to work in the underground sweatshops of the future while society is being rebuilt to their specifications. Similar to the Star Trek TOS episode “The Cloud Minders.”
The Shroud of ThelmaLou. 