I created a 3d human face with 'Facegen Modeller' and I opened it with Blender, trying to adapt it to my needs. Inside the eye there is a bluish/greenish oval shape. What is that? When I try to mark it as seam, the color does not change. I removed that shape in edit mode and then created it again and now it seems to be marked correctly. Am I making a mistake?
You marked those edges as being sharp, possibly by accident.
It happens when you select an edge or edgeloop and press Ctrl + E > Mark Sharp
To remove it select the edge or edgeloop in question and press Ctrl + E > Clear Sharp