The backlash to season one of HBO’s “Girls” erupted as soon as the wunderkind glow around Lena Dunham had dimmed. On The Hairpin, Jenna Wortham lamented Dunham’s “failure to weave a main black character” into the show.” The blog Racialicious posted a piece titled “Dear Lena Dunham: I exist.” And Dunham leapt to apologize.