Learn Croatian: Nemoj dirati žulj ako te svrbi. - Do not touch the blister if it itches.

Both mean "do not touch" and are completely correct. In Croatian, you can form negative commands in two ways: using the negative particle ne plus the imperative (ne diraj), or using the helper verb nemoj plus the infinitive (nemoj dirati). The nemoj structure is extremely common in everyday speech, especially for giving advice or general prohibitions.

Is the noun žulj (blister) in the nominative or accusative case here?

It is in the accusative case because it is the direct object of the verb dirati (to touch). Because žulj is a masculine inanimate noun (a non-living thing), its accusative form looks exactly identical to its nominative form, which often trips learners up.

How does the verb svrbi (itches) work here, and why do we use te?

In English, you say "if you itch" or "if it itches". In Croatian, the thing causing the itch (the blister) is the subject of the verb, and the person experiencing the itch is the direct object. So ako te svrbi literally translates to "if it itches you". The word te is the short accusative form of ti (you).

Why do we use the imperfective verb dirati here instead of its perfective pair dirnuti?

Negative commands (telling someone not to do something) almost always use imperfective verbs in Croatian. This is because you are forbidding the action in general ("don't be touching it"), rather than focusing on preventing one single, completed instance.