Atmel SAMD21 DMA Transfer Complete Interrupt not firing
The block transfer control register has a field BLOCKACT that indicates the action to take after each block. After configuring that to INT (Channel in normal operation and block interrupt) the DMA kept operating continuously, while triggering the ISR at end of each block transfer.
I had to add the following line to the configuration of each of the two descriptors. 

descriptor_config.block_action = DMA_BLOCK_ACTION_INT;