A ship is a ship and gets insured as such. I'd expect the insured value of cargo to be at replacement cost. Until the slave trade abolition act of 1807 slave ships and their 'cargo' were legal and could be insured1.
After that act was passed slave ships were treated as pirate ships and dealt with as such. The Royal Navy after Trafalgar didn't take much notice of the national flag that ships were flying, any ship was subject to being stopped and inspected.
1 Lots of factors are taken into account (possibly even ethics) that can make insurance uneconomical, possible slave uprisings on ships may have been viewed in a similar manner to a modern teenager asking to insure a Ferrari.