I looked for a biome conflict between BoP and Thaumcraft. I didn't see conflicting IDs. Unfortunately, I was in a hardcore world and lost the world when I died.Sinister nodes do generate eerie biomes around them, so little patches of eerie here and there is normal.
But you say you found an entire normal-sized biome made of eerie biome? Strange.
I wonder if you've got a biome ID conflict between TC and BoP or something.
It was displaying 'Eerie' as the biome when I hit F3. Also, silverwoods were spawning in there. It was strange, that's for sure.You can find sinister nodes in the wild, that aren't in totems or other structures. But this will still only make a patch of Eeriee Biome. There is an Omnious Forest Biome in BoP, which is similarly darkened. Without a better description or a picture I can't say for certain though.