Well, all three DK specs were designed to be viable for tanking all the content.
It is true that Blood is the preferred raid tanking spec - but that’s just how it washed out - mainly due to their superior passive mitigation in top end gear (Frost is very close but lacks the self healing).
For dungeon and heroic tanking they’re all viable. You can even raid Tank effectively as Frost (and players actually did back in the day). It was so close that there was even some debate as to whether Frost or Blood should become the dedicated Tanking tree for Cata.