Oh hell. You’re probably right. So much for my theory.
Great pick-up ice breaker. I’ll add that to “So, do the curtains match the rug?”
Yep. All this. Also to deter larger carnivores and for juveniles to be less likely to be bothered by older members of their species. Showing off your vivid white teeth is often a way to avoid a fight. Sometimes if you look ferocious enough, you don’t have to fight, which means you can save that energy for hunting or producing more offspring.
Most animals in fact will do whatever they can to avoid a fight by displays and posturing. Fighting is only a last resort.
I like the “show off your teeth” hypothesis. Unfortunately, a lot of questions about evolution of traits come down to “just-so” stories that sound good but are difficult to test. I like hearing the stories anyway.
I don’t think we’re getting to an asnwer…
To clarify, I wasn’t interested in humans, bipedalism, buttocks, or breasts. (For the purposes of the question, that is.) I did make the offhand remark about humans having pink or pinkish lips, but that was only to contrast human lip color with dogs. There may be some humans with less than pink or pinkish or red lips, but it’s besides the point. The question remains “Why do dogs have black lips?”
Our dog is a cockapoo, a poodle-cocker mix. His fur is principally white, with buff (tan) spots. Before that, we had a cocker who was all buff (tan) furred. FWIW, both had skin under tha hair that was pinkish spotted aparently randomly with dark tan/brown splotches. This seems to my observation to be rather common among dogs. As do the black lips. And when I say, black, I mean black like rubber O-rings. Black like black ink, not dark brown, not anything else. Actually black, not some euphemism from (human) racial naming conventions.
If it had something to do with capillaries, then it seems to me that their lips would be some sort of variation of red, but they aren’t.
Sun exposure wouldn’t get it either, because both our dogs were/are indoors way more than out, and besides, the setup of their faces keeps their mouths in the shadow of their muzzles rety much continuously.
It’s speculation, but what’s wrong with the “to show off their teeth” answer?
Of course, the correct answer is “I don’t know.” 
somewhere in my head (probably the gutter portion) my brain mentions that natural lip color often matches nipple color as well.
My Golden one day had a big pink splotch on one of his jet black lips.
Vet said something like “color flakes off sometime.” Have no idea how that works, but it should be noted for the purposes of this thread.