The dam being blown up didn't flood Crimea. It flooded downstream Russian controlled areas. What the dam being blown up DID do was limit the water from the reservoir that can flow to Crimea.

Prior to the invasion, Ukraine had walled off the canal to bring water to Crimea. This was one of the express goals of Russia's invasion - to restore the flow of fresh water to Crimea. Crimea had been suffering massively under water shortages for years before the invasion.

This alone would be a reason for Russia to NOT blow up the dam. Their largest warm water port and biggest naval base depend on water from the reservoir and that water flow is limited/endangered due to the dam being blown up.