Would a black sailfin blenny be good with a seahorse?

[rayjay]

Well first of all, beauty is in the eyes of the beholder. Otherwise, our tanks would probably all look the same.
Stinging corals are a problem with seahorses and so it's important to keep the TANKMATES GUIDE in mind when planning your seahorse tank.
Keep in mind also, the requirements needed for your tankmates selection as some don't handle lower temperatures or need extremely high lighting which can cause temperature issues.
Some of us don't keep tankmates of any kind, or at least keep it very minimal as the fewer tankmates, the less the chances of introducing pathogens that can affect your seahorses. I don't even keep live plants myself.
Anything you can do to minimalize problems increases your chances of success. Even doing all that you can does not guarantee success though so keep that in mind.
You have to be more diligent about everything especially husbandry to give yourself the best chances.