You are confusing the talent trees with the presences. For example. For tanking, most of the talent points go in the blood tree. But while tanking you switch to frost presence. Frost presence causes less damage taken and more threat.
Dps puts most of its talent points in the frost tree but when fighting they use blood presence.
Originally, all three specs were capable of tanking by just switching to frost presence.
Its like a warrior switches into defensive stance or a druid goes into bear form. That what frost presence is.
Frost and unholy are typically your dps specs. Blood is your tank spec/ideal leveling spec, although any of the above work well enough for leveling too