Yes! While all three specs are technically able to be played in a Tank role, Blood is considered to be the strongest due to its immense amount of self healing and its ability to scale better with gear than either of the other two specs. However it is not uncommon to see someone tanking as Frost. Though Unholy does not have as many benefits as the other two specs it is still technically able to tank, however it is not advised without an extensive amount of knowledge on the class and the spec and is oftentimes just worse than both Blood and Frost.