I've been experimenting with different builds hitting target golems in Heart of the Mists when I first noted this. Before starting recording videos and submitting bug reports, I first wanted to make sure I'm not missing something here.
The problem is when I appy skills which set enemies on fire with this trait enabled, some of those skills correctly blind enemies, while others blind them, yet not all the time, and not at the same time, for some reason (judging by the "dark cloud" thingy which appears above the blinded foe); I only tested with dagger/dagger weapon set, using the place on the map when golems stand in a big group close to each other (so each AoE skill was hitting 5+ of them). Here is the list:
Trait's description reads "Blind foes that you burn" (note the plural form), also in wiki I see this: "If multiple enemies are burned by the same hit (such as with Flame Burst) they will all be blinded. Attacks that burn several times, such as Burning Retreat or "Feel the Burn!" will only blind the enemies struck by the first hit." So I assume behavior I observe in case of "Signet of Fire" and "Ring of Fire" is correct one, and others are bugged.
Any explanations to this, perhaps?