Acting like a complete idiot. Presumably "the blue crack" is Drain-O Crystals. "Have you been smoking the blue crack?" is thus analogous to "Have you been drinking turpentine?"
Note that this usage pre-dates the use of "blue crack
" as slang for Tylenol PM.
"A customer asked me today if I could see her computer screen through the phone. I asked her if she was smoking the blue crack."