Is this a legit term or was it invented by hipsters? What bands are considered blackened death metal?
I think it's application is generally via commoners. It refers to a style largely revolving around Dark Funeral styled music, or generic death metal music with some tremolo riffing akin to black metal.
However, music that's like Hail, Profanatica, Goatlord, Throneum, etc can be referred to as black/death metal as well, but that's usually because their music lies within the boundary between the two genres. I think these bands generally are of the understanding that death and black metal are two sides of the same coin, whereas the blackened death metal bands discussed earlier (Behemoth et al) are bands that consider the two genres to be mutually exclusive.