Joiner put the black backboard on our kitchen round a corner but
didn't mitre. Is there any such thing as a black sealnt so I can put
down the join to cover the wood thats showing?

Black silicone sealant is available at most larger DIY stores and Internet
suppliers.

i.e.

http://www.decoratingdirect.co.uk/viewprod/s/SIRSSHMA/

Christian.