Technically no, you shouldn't do it... To my knowledge, painting clear receivers black constitutes some form of manufacturing replica firearms. Hence why none of these retailers offer it as a service. That being said, if you want to do it, don't do anything stupid with it and draw attention to yourself. The newer tinted ones with black mechboxes are great. You can't even really tell with the mag in the receiver is clear unless you're nice and close.
They're clear to begin with for a lot of silly reasons with our laws. If you're really interested, do a search here on ASC, there's been lots of discussion. Basically if the receiver is clear, they're allowed to be imported legally without special licenses.