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.
DK’s in classic are designed so that to tank or dps, you will have to dip into at least two trees. If you scan the trees you will see both dps and tank talents in all three trees. So it’s just a choice as to what kind of tanking do you want. Frost is definitely fine as a tank, but you will probably be dipping into blood or unholy for some talents to round out your build.
If you’re planning to prog raids, you want Blood. If dungeons are the only concern, Frost is fine
Reason: Blood is infinitely better at sustained mitigation so it’s way more desired for long raid boss encounters. Dungeon encounters, on the other hand, are never long enough to warrant the extended sustain so the extra damage from Frost is desirable for quick clears
You’ll be able to tank as frost. People who say blood is infinitely better, underestimate frost. As someone who tanked and raided in Wrath, frost is the spec you want if you’re willing to put the effort in. Healers and dps loved it when I tanked because I specialized in avoidance. Get to 640 def, 20% dodge, 20% parry, and then max out your hit (yes I was dual wield). You’ll get more haste as you go as well. You can be pretty OP with in PvE until ICC (you need health food, frost resistance gear/buffs to compensate), and PvP. It’s probably why Blizzard went with blood after because frost had multiple uses and can be very OP as a tank. It was also very fun to play and you know what happens to fun… a nerf!
