Kitten is looking a bit ill?

We have 5 just-turning-6-weeks-old kittens in our house, and one of them looks a bit off. Hes got black discharge around his eyes (just at the corners) and in his nose. And he's normally one of the more boistrous babies, but he hasn't been as active lately. We're going to take them in soon for their shots, but this concerns me. Is it just an allergy, or a cold, or what?