Ah, but anyone who knows fashion knows that not all black socks are created equal. There are the black knee socks, the short little black ankle socks, the ribbed black socks, the "nearly grey from being washed so often" black socks, the "holes in the toes" black socks...
In my opinion, the question should specify that all black socks are homogeneous and all blue socks are homogeneous.
Then again, when was the last time personal opinion mattered on the GMAT?
(Long story short: assuming all black socks are identical and all blue socks are identical, probability is 1.)