mine get along pretty well with each other and also the oscar and red devil and jag that are their tankmates... they had a few nit picks when they were growing up.. but as mature BB's they are fine.
Blackbelts are the smartest fish I have ever owned. Impossible to net the fish as they are the best escape artists in the fish world. It used to take me 20 minutes or more to catch the last one I had that was around 9" long. Super smart fish.