Horizontally-striped blue/black fish (such as maingano or male johanni's) will probably not incite a demasoni (for the conspecific aggression to kick in the demasoni has to mistake the other species as a demasoni and the different stripe patterns should greatly deter that).
A male pseudotropheus saulosi or labidochromis sp. mbamba would be a more risky selection:
This yellow top mbamba was a frequent target of demasoni aggression in my tank, it would shift to near solid purple colors to avoid their wrath.
Horizontally-striped (or similarly colored) members of the metriaclima genus (the zebra complex) usually have no problem deterring demasoni's as they're usually more aggressive and larger. The species depicted in my avatar (black barred zebra) and this male msobo dominated the 20+ demasoni's that they shared their tank with:
With that said, adding new mbuna's to an established set up may result in some aggression, so you might actually see some temporary territorial aggression from the maingano's toward the newly introduced demasoni's.