I've experienced the "black hole" effect once.  It was disturbing because I didn't understand it.  My Reiki Master/Teacher and I were working on a woman, and I just saw and felt a blackness.  Not evil or anything, just there.  I will add that the blackness did not affect my hands or anything, I could just feel/see it in her.  After the session, my teacher explained the woman's background.  She had recently had a miscarriage and was still emotionally distraught.  So even though our energy didn't fill that void with white light or any light, energy was passed on.  Maybe she later accepted it.  I don't know.
My point is, maybe your client just has some voids that will or will not accept your work.  The blackness on your hands, my suggestion is to shake it off into the earth to rebirth.  When I do pick p negativity that clings, I literally flick and shake it off, downwards into the earth for it to absorb and rejuvenate.
good luck.

i agree with GEM, BUT...... if you still want to use white light then i wud suggest you to first sponge the patient with saline water throughly. then clean the whole body aura with electric voilet or flame color. once cleaning is done then try to give white light. but before doing this, first examine what are the problems to the patient. DO NOT TRY TO FOLLOW SIMILAR PATTERNS OF TREATMENT ON EACH PATIENT.