import org.eclipse.emf.transaction.RecordingCommand;
import org.eclipse.emf.transaction.util.TransactionalEditingDomain;
import org.eclipse.emf.transaction.util.TransactionUtil;
import org.eclipse.uml2.uml.Class;
import org.eclipse.uml2.uml.Stereotype;

...
TransactionalEditingDomain domain = TransactionUtil.getEditingDomain(yourPackage);
domain.getCommandStack().execute(new RecordingCommand(domain, "Create a block", "Some description...") {
	@Override
	protected void doExecute() {
		Class yourBlock = yourPackage.createOwnedClass("yourBlockName", false);
		Stereotype blockStereotype = block.getApplicableStereotype("SysML::Blocks::Block");
		yourBlock.applyStereotype(blockStereotype);
	}
});