I have abunch of stuff that was recommended to me by a local metal had here now that I"m getting into black metal, some of it seems a bit obscure but I've hunted down copies and will be buying them on the 21 of August... if any fo them are heavier than the usual B.M will post them here...
Try not to necro threads nearly 10 years old mate.
That said, the heaviest black metal band without going too far into black metal that crosses over to death metal or thrash metal is Mayhem in my opinion.