Dominican citizens do need a visa to enter the Schengen area but just about any Italian visa should be good for that. You will enter the Schengen area and get your passport stamped during your layover in Frankfurt. For this reason, it's important that the validity of your Italian starts before the date you will land in German (and not merely when you would enter Italy) and covers the whole stay including any layover on the way in or out of Italy.
