From the IMDb FAQ:
Are the Bloom brothers really brothers?
Yes. At the very beginning of the movie, a caseworker can be seen opening a file cabinet and pulling out the files on young Stephen and Bloom. The file is tagged "Bloom (2)", suggesting that there must be a "Bloom (1)".
Does Bloom have a first name?
The only reference to what Bloom's first name might be comes from a con in which Stephen calls him "Victor." Whether that's his real name or only a contrived name for the con is unknown.
So, it's entirely possible that Bloom wasn't really Brody's first name, but his last name. This would make the title accurate.
I suppose it's not uncommon for people to become known more by their last name than their first. Also, Bloom is certainly not a common first name.
(This question reminds me of the Mario bros., Mario and Luigi.)