This is just my interpretation.
Blood is the life essence of the being.A blood Dk can suck this life essence to keep himself alive,thats why they are the tank spec.
Unholy deals with the Flesh and undeath,they can destroy the enemy with diseases and turn then in into mindless unatural beings.
Frost is -or should be-the cold of Death and deals with souls,their icy attacks also pierce the individuals soul while also have the hability to freeze others in place.
The problem i see is that they push the Frost spec too much to the Frost side,looking more like a warrior that uses frost magic then a actual Death Knight that attacks the enemy soul and also use Frost attacks.
again,this is my interpretation.