75 gallon is the minimum if you want them there for their whole life, but a 55 gallon could work as a temporary home for a lone blood p. for some time.
mine is in a 40g breeder and for its size, it is generous. i had it for 6 years. i would say a 55g is fine. a 75 would be for something like a red devil.
Go with the 75. The BPs I constantly see in the LFS love to get a good run at the powerhead current and this would give them plenty of room. Plus you would have some room for a few dithers that would make the parrots more comfortable being out in the open.
My friends roommate has two of these in a 10 gallon with a smallish cichlid and some sort of cory. The water is really hazy and one of the fish looks like something is wrong with its mouth. The fish don't look good at all!