Personally I still consider them to be black metal.
Keep of Kalessin, Catamenia, Old Man's Child, Blut Aus Nord, Astarte, Rotting Christ and a lot of Emperor and many other bands are bands who aren't even black metal (in some cases anymore) but are still regarded as such. If Dimmu Borgir weren't as popular to in regards to the more mainstream side of metal, they wouldn't receive the amount of flak they do and there wouldn't be this issue whether or not they play black metal. I'm not saying they fit completely into the genre either. I mean when people talk of Burzum, nobody jumps out and says "Burzum isn't black metal, used to be but later changed to ambient."