I’m sure it has something to do with the fact that I mirrored them (using ctrl + M) but I don’t know how to fix it… I tried ticking DoubleSided in roblox studio too but nothing happens. Does anyone know??
I do not know much about blender or meshes in general, but to me, the darkness on the right side of the tentacle monster in the game looks like a lighting problem. Have you tried changing the in-game lighting or just moving the part to confirm this?
Once again, I could be wrong. Anyway, I wish you luck.
Have you tried selecting all the faces of the Mesh in Edit mode in Blender and clicking the Mesh > Normals > Smooth Vectors (or try the Reset Vectors tool above or below it).
It could be that mirroring the Mesh caused the Vectors to be modified which changes how the light is displayed on the face.