black smokes "normally" indicates fuel related

blue "normally" indicates oil

how old is the engine, new build or old?

if its old, i'd bet its worn oil rings on pistons. it would make sense as when old the pistons have expanded due to heat; therefore oil and pass by the worn oil rings, when the motor gets hot, the rings/ pistons expand and seal better

the only other mechanical thing it could be is worn valve stem seals or valve guides, a good way to see if its stem seals/ guides is to overrun down a big hill; then at the bottom give it a boot - if a big puff of blue smoke appears, then "more tha likely" its likely to be guides or stem seals.

to be fair it could well be oil sitting in the mainfold or it may well need a decent breather, but in my experience i doubt it.

has it been set up; have you had the CO% set correctly? what's the AFR reading? what chip is it? it could be a rubbish chip which just overfuels or the wrong chip for your spec

Matt