Would a black sailfin blenny be good with a seahorse?

[rayjay]

Unfortunately the odds are that you won't fine someone that has tried that particular fish on this forum.
Best to go by the "orgs" tankmate list and if it is acceptable there the odds should be in your favour.
Of course, there are fish not recommended that sometimes work out, just as some that ARE recommended do NOT work out.
Just like reef fish compatibility, no 100% guarantee.