This is not totally uncommon in the metal world, Dimmu Borgir, Fleshgod Apocalypse, Septic Flesh, Behemoth, and many others use orchestral sounds to enhance their music. None of these bands are black metal though, and that's what makes Carach Angren special.
Black metal as a whole tends to be a very harsh sound in general, so hearing it with pretty melodies in the background can be disconcerting.
This band also tends to make themed albums, and the album that this song is on is based in WWII. Just as a heads up.