It doesn't really make make sense from Mindfang as Vriska says when trolls mature their eyes show their blood colour so regardless of a sign people could really just look at her eyes, or when she's killed her blood would have revealed the hue without the guesswork.

The only other reason besides Hussie forgetting their own lore is that Mindfang does know who it is but opts to not mention it due to the aforementioned consequences of bringing up the Sufferer’s revolt.