What about extreme melodic metal like EToS called themselves? Death metal is a wrong word to describe Kalmah's music and so is power metal...it's like defining Children of Bodom with one genre.
But if I must choose between death and black, then it's death metal. As mentioned above, melodic black has more minor-like, simplistic and repeatitive riffs, more scary feeling, more complex song structures, not many solos and the vocals are higher pitch and they often sing about satan or something. And if you listen to Kalmah...it's the exact opposite.