Block sizes vary wildly, so I can't just always round down, but it doesn't seem like the google maps API understands these kinds of locations.
I considered trying to go half way: for example, get the location of 5050 Q Ave for the 5000 block of Q, but blocks are not a regular interval either, nor is spacing on a block constant. I'm pretty sure the service doing the locating will need to be aware of the actual blocks to return a good location for this kind of query.