Heh! My wife and I (she's white, I'm black) unintentionally did this kind of testing in the Boston rental market in the mid 2000s. It was pretty disheartening (the weird thing is that once we stopped using real estate agents and only looked on Craiglist for rent by owner things were much better).

This sounds like a whole other level and that there are no actual laws (I mean, landlords use all kinds of tricks in the U.S, but no landlord is stupid enough to say "Sorry, I don't rent to blacks")