In my opinion Borknagar is a "Green Metal" band.
I invented the label "Green Metal", i don´t know if anyone had this idea before me...
Green Metal, in reference to black metal (the color), because they have strong black metal influences in the music and the texts deal with nature (green).
Pagan or Viking Metal might be a partly right, too, because of folkish influences and the mix between "black vocals" and clear vocals.
But the texts don´t deals with typical pagan or viking metal topics and the folk influences aren´t as strong as in pagan/viking metal bands.