Well what gives?
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In the days before CAD, before Xerox even, plans were copied by an early photographic process known as Cyanotype, invented by Sir John Herschel in 1841. Cyanotypes are blue.
Because they absorb all other wavelengths of light besides blue.
Over fifty posts! Yeah! Woopie! Wahyoo! Top that man!
Zebe, take a look to the left. 
If they were any colour other than blue, they wouldn’t be blueprints would they?
You don’t see real blueprints (white lines on a blue background)much anymore. You do see blueline drawings (white background with blue (or another dark color) lines)quite a bit. A blueline machine most commonly uses a mylar film or velum film image and a sheet of paper that has been coated with a substance that is reactive to ammonia. You lay the mylar over the sheet, feed it through a set of rollers that expose the sheet to the ammonia vapors (fixing the outline onto the sheet) and then another set of rollers that expose the sheet to a “black” light (which exposes all of the other areas of the sheet except the outline). Paper goes in colored (ususally yellow) and comes out white except for the image, which is in blue lines. They are called Diazo machines (or at least the ones I use are) and the ammonia is incredibly strong (much, much more so than household ammonia). I made the mistake of knocking a bottle of it over in the office and the whole building had to be evacuated, the hazmat team called, etc. But that may have been more than you ever wanted to know about blueline machines.
Would that be like a ‘ditto sheet’?
Boy think of that! One huge ditto sheet, I would just wrap it around my head for hours.
You don’t see real blueprints (white lines on a blue background)much anymore. You do see blueline drawings (white background with blue (or another dark color) lines)quite a bit. A blueline machine most commonly uses a mylar film or velum film image and a sheet of paper that has been coated with a substance that is reactive to ammonia. You lay the mylar over the sheet, feed it through a set of rollers that expose the sheet to the ammonia vapors (fixing the outline onto the sheet) and then another set of rollers that expose the sheet to a “black” light (which exposes all of the other areas of the sheet except the outline). Paper goes in colored (ususally yellow) and comes out white except for the image, which is in blue lines. They are called Diazo machines (or at least the ones I use are) and the ammonia is incredibly strong (much, much more so than household ammonia). I made the mistake of knocking a bottle of it over in the office and the whole building had to be evacuated, the hazmat team called, etc. But that may have been more than you ever wanted to know about blueline machines.
Yeah, bluelines.
I haven’t even seen a blueline since college. Does anybody still use this process pinnr described? Every company I know of now uses CAD, with E-size plotters and E-size xerox machines.
Now you’re bringing back memories. When I started working as an engineer back in '82 we still drew all our schematics on velum with pencils and plastic templates. I still have a logic template in my desk drawer. No one knows what it is.
The first system I designed had 78 pages of D-sized schematics, all hand-drawn. Running a set of copies meant feeding the 78 pages of velum through the blueline machine, one pass to expose the sheet and another pass to process it. SLOW, and extremely stinky. Thank goodness for CAD.
Kevin B.
We had one in my high school drafting class. We mainly used it to make blue “edge cover thingies” (goes over the staples on the edge of a set of papers) by copying a clear plastic strip with all except the lettering blacked out, for a result of white lettering on a blue background. I’ll post a pic of it in a few minutes.
See the aforementioned blue border. I love the little picture on it.
We did the actual drawings with AutoCAD 14 and an E-size HP Designjet plotter, BTW. This was last year.
Oh yeah, the little blue and yellow cards attached? My “district–qualified for state” and “state–first place” rating cards 
(99-00 SkillsUSA/VICA State Competition, Display, Architechture)
Because “red prints” sounds funny.
The printing industry does, quite a lot.
In addition to a color match proof (actual color as it should look when printed), a blueline is made with each of the CMYK plates at a slightly different “tint” of blue, and trimmed & folded exactly how the final piece should look. With the 2 proofs, you know what to expect.