July 23, 2004, 10:42 a.m. Def to Reason HBO looks deep into an abyss. By Mark Goldblatt If you dive head first into the cesspool of black urban culture, through the flotsam and jetsam of bling-bling jewelry, designer sneakers, and "Free Mumia" T-shirts — through the Ebonic endearments of "ni**a" and "ho" and "dawg" — and if you struggle down past the snarling, muttering studio-menace of gangsta rappers and the haunch-spreading, butt-bouncing images of "empowered" womanhood — and then if you plunge deeper, past conspiracy theories about the LAPD and O.J., about CIA agents and crack, and about Jewish scientists...