Earlier

Yes, I see what you mean. A sentence can be either a declarative or an interrogative, but not both at the same time. 'Declarative questions' are spoken only for expressing surprise, disbelief etc, and is understood from their intonation.

Have I not understood until now?

Here, the subject has come after the verb, and so it cannot be a declarative sentence. Is it called an 'interrogative negative'?

Why I haven't understood this until now?

Here, the word order is subject-verb-object and though a question word and mark is present, it is still a declarative sentence -- a declarative question at that.

CalifJim a final 'till/until now' works with the present perfect in the declarative negative.

I can say, a final 'till/until now' works with the present perfect in an "Interrogative negative" as well.

I have not understood this until now. (declarative negative)

Have I not understood this until now? (interrogative negative?)

Why I haven't understood this until now? (declarative negative)