Dark eye mucus = allergies?

I noticed the other day that the normally clear mucus in my cats eyes is a little dark. Otherwise, he is acting fine. Eating, bathroom, sleep, nose temp - all normal. Since the weather is getting nice he has been outside a little in our fenced area. Could his dark eyes be a reaction to pollen or spring allergies? I know it's a weird question, but he isn't showing any other symptoms of being sick.

Are you seeing darker fluid actually in the eyes, or is it just a darker stain on the fur at the corners of the eyes? I have one cat whose tears dry a dark red. The vet told me that's just the normal color of cat tears.

No fluid or drainage, and it isn't swollen. Just the inner "eye lids" are darker than normal.