Um where? The only place I find anything that say blacks are rare are when they are discussing the possibility of non-fading black - they say those are rare. They are NOT talking about regular blacks. Here is the quote. If you find another one on the site, please post it because I don't see it.
Quote from site:
"Some older books have theorized that there is a third allele, called ED, dominant to the other two, which would explain the "non-fading black" which is rare, but occasionally seen.
However, more recent studies have discounted this theory. It is a fact that some very few black horses do not fade in the sun, but it is unknown at this time what genetic mechanism causes this. One theory that has been conclusively disproved is that the "faders" are Ee, and the "non-faders" are EE. Now that there is a test for the "E" genes, plenty of blacks that fade terribly have been identified as EE, so that can't be the answer."