Blank is an adjective here. Usually, adjectives go before the noun, but not always. A basic English course only covers a subset of English grammar, so you will encounter correct English phrases which are outside the basic grammar that you have learned. My intermediate grammar book for students is about 200 pages long, mostly explanations and exercises. I have a grammar book for teachers which is about 700 pages long and I have a grammar book for linguists which is about 1500 pages long. In your text "Finally, don't leave an answer blank," blank is a resultative adjective. Consider these examples: He painted the old fence. [old = adjective] He painted the old fence white. [white = resultative adjective] [meaning - He painted the old fence. After he had finished, the fence was white.]

In that construction, it's an adjective.

If a painter asks you what color to paint your house, you might reply "Paint it green". ("green" is an adjective that modifies the direct object "it") Grammatically, "paint it green" works the same as "leave it blank". If we said "Paint green the house" it would sound strange, as if the adjective "green" were trying to modify the verb "paint". Also, if we said "Paint the green house" that would say the house is already green and would say nothing about what color you want to paint it. If you wanted to change a yellow house into a green one, you would say "Paint the yellow house green" in which "house" is the direct object and "yellow" and "green" are both adjectives that modify house. This example clearly explains why "green" needs to go after "house".

There are great answers to your question already but I wanted you to know that ‘answer blank’ can also work as a noun, as you suggest. Fill in the answer blank. Write the correct answer directly on the answer blank. I’ve made a test by replacing the verbs with answer blanks.

It's an adjective; in the imperative form 'Do this', 'Don't do that', etc, as in instructions, it's common to place the adjective after the noun.