Well, say you and you partner carry blue-green gene for eye colour (as in green from one parent, blue from other), then the green will dominante blue so you and your partner will have green eyes. However, your offspring, inherits one from its father and one from its mother, so has a 1/4 chance of having blue eyes (i.e. you both have heterogeneous genes, your child has homogeneous). This is put very simply, otherwise it involves a lot of typing!
This is based on there only being two genes that represent eye colour. It is believed there are probably more, as they can't at present explain all eye colours fully.
Also, children's hair is often very light and develops colour as they get older.
The child could therefore very well be yours and I wouldn't worry. If you have another reason to think it isn't, other than it's appearance, get a paternity test (that's assuming this has happened and this is not theoretical). Genetics are complicated and strange things happen all the time.