The confusion with the description of Frédéric Bourdin

This is such a good point that everybody seems to fall speechless... I myself have nothing more to add.

He said that he'd given a description that was vague enough for the person on the other end to have many options, he didn't say that he'd mentioned his hair and eye colour. A few speculations on how he might have pulled it off.
For example, he might have said the the kid (he) had dark blonde hair so as to have them include children with blond to brunette hair, or maybe the person he was speaking to only had a black and white picture themselves and the coloured one he got later was from a different file or whatever; he might have said he was fifteen to eighteen years old (saying the boy himself wasn't sure anymore because of being traumatised), that he was American, and maybe that he had been away from home for three to five years. He could always say that the boy wasn't speaking much so that the people on the other end would have had to fill in the blanks by asking more detailed questions, because they would have been determined to help and find out who that kid was. Hell, he might even at that point have told them that the boy had spoken of his kidnappers having changed his appearance.
Based on that information the other person would have narrowed it down, and in an effort to narrow it down further might have asked things that would actually give Bourdin something to work with and incorporate, like 'Ok, now have ten boys that seem most likely, let's see if we can exclude some more of them. Does he have a tattoo on his hand?', which he would've affirmed because it was something that he could change.

So in conclusion, I hope you realise you're reading my signature.