I used to have real problems with the black / grey wash on my back piece - it got all lumpy and burned like a fresh tat.

I've had it gone over in a colour wash now and that's made a huge difference, not more problems. My tattooist says its a common reaction to the black ink but its not dangerous and won't ruin the tat or cause any problems. Just use a heavy duty moisturizer on it and take an antihistamine to reduce the reaction.