At first, I found Quite Black a funny name - I'm not sure why, but the understatement of it seemed amusing. But! Google is American and there is definitely some difference in the meaning of quite in UK vs US usage. The latter two make me think that Google intends quite in its meaning of very or completely. We can use it like that here, although it sounds a bit old-fashioned or posh:
That's quite enough of your nonsense, young lady! I'm quite sure you wouldn't speak like that to your mother!So it isn't an understatement; they mean that it's very black, or completely black. Disappointing.
Lynne Murphy has (of course) written about this difference, here and here. She points out that the difference is 'very much' (AmE) vs 'not so much' (BrE), or that it strengthens the force of an adjective in American English but weakens it in British English.
I don't know know quite how I would characterise it in British English. It does weaken the force, and it does mean 'not so much', but so much, as always, depends on the tone. How you say it can make It's quite black mean that it's waaaaay too black (sort of rise-fall intonation - I'm not a phonetician, sorry) or that it's really a very good level of blackness (rising intonation). Of course, this is what intonation does, and to some extent you get this without quite, but it adds this ambiguity. In both cases, it means that there is a high level of blackness. With yet another intonation (strong emphasis on quite), it can also mean that it's relatively black, but not as black as you would hope.
Either way, even though the 'high level of blackness' meanings are very accessible in British English, we interpret the phrase Quite Black as an understatement, a weakening of the force of black, and not as a strengthener.
If I was Google, I'd have released it in Purest Green: