Superlative or not?

I don't see a problem with "full-blown" used this way. However, some aspects of the sentence make me a bit uncomfortable. I'll base my answer on this version of your sentence:

In a time of full-blown financial crisis, stringent measures must be taken.

In case the commenters are still uncomfortable, please notice that the following sentence follows the same pattern:

In a time of imminent complete financial collapse, stringent measures must be taken.

We see there is an either-or situation. We're either in a time of full-blown financial crisis [or imminent complete financial collapse] or we're not. The assumption is that we are. (Analogy: someone is either pregnant or not pregnant.]

So now to answer your question:

If we take "superlative" in the grammar sense, then no, it isn't. Grammar superlatives are things like good - best, serious - most serious.

If we take "superlative" in the more general sense of "An exaggerated or hyperbolical expression of praise" (Oxford) then I suppose it sort of is, although usually superlative in this sense refers to positive qualities, for example "After writing nearly 100 DVD reviews, one begins to run out of superlatives to describe the better discs which pass before the eyes and ears."