I have been playing classical piano reading sheet music, and I want to begin blues by similarly learning some pieces from score. What would be some good books/sites I can use for this?
This question comes across as a shopping recommendation, but since we don't do those on Music.se I'm going to stop short of recommending specific books.
Blues is a broad genre of music -- for example the Chicago Blues of Howlin' Wolf is quite different from the Delta Blues of Leadbelly or the Texas Blues of Stevie Ray Vaughan. We don't know which of those you're interested in sounding like.
You probably have a specific piano sound in mind, so find a book containing a score of a song in that style.
I searched for "blues sheet music" in Amazon and there were plenty to choose from.
Note that blues musicians rarely use sheet music (my guess is that most can't even read it) - it is an improvisational tradition. There are books on improvising blues piano too. Amazon's search reveals them just as easily. Use the reviews, and the contents listings, to decide which book covers the kind of sound you aspire to.
With blues and jazz, there's really not the concept of a "standard repertoire" of the kind recognised by classical music students. This isn't really because the form is young (it's had 100 years or so, and developed from folk, after all) but because it's so broad and varied and, as I've said, improvisational.
There is the concept of the "blues standard" and the "jazz standard". A jazz soloist can show up at a club and tell the house band "Rainbow in A", and they'll play "Somewhere Over The Rainbow" (without sheet music) for him to solo over.
However that's just a melody and a chord progression; not an arrangement.
For an example of this, search YouTube for "Mojo Workin" and watch a few of the videos of Muddy Waters performing it. Notice this is the same band leader, and in many cases the same backing musicians, but no two performances are the same. Different tempos, parts assigned to different instruments, different grooves; and it's likely these arose organically at each performance.
... then look at other people performing the same song, and see how their versions are completely different too.
You don't really learn a blues piece. You learn a collection of blues patterns. You learn how to improvise your own in a similar style. You assemble these into a performance of a song; you do it differently each time.
Blues is rarely played or or written using scores, at least for a complete song. The chord structure and common scales are quite simple and there is typically a lot of improvisation on top of it. This makes "Blues jams" so popular, you can easily sit in with people you have never played with and songs you may have never heard. So while it's a perfectly good start to learn and play transcribed scores, you may not actually get fully into it unless you move without scores. It's most of the fun anyway.