my black garlic cooking in a rice cooker went brown and hard before black any ideas?

To make black garlic, you're trying to break down the sugars in the garlic. It sounds like you're dehydrating it, rather than giving it time for the enzymes in there to do their work. You need to somehow prevent the moisture from escaping. Covering the vent hole may be sufficient, but I'd suggest putting the garlic is in an airtight container to be sure - perhaps wrap it tightly in plastic wrap to keep the moisture in there. Once the sugars have converted, you could then unwrap it for a little while to dry out the skin further if you liked.

There's an interesting article that talks a bit about what's going on inside black garlic here

I've been making black garlic in the rice cooker successfully by wrapping a bundle of cloves twice with tin foil, I also close the vent of the cooker and put some glad wrap around the lid, mine take around 14 days to get black but still very moist so I just unwrap and leave in the cooker unsealed until they dry out a bit