Ive never done that before but guessing quite a tricky job, definitely food for thought though.I say open them up and spray the inside bits black so halfy halfy! That's assuming you are talking about blacking out the lenses? I think it's a bit much personally but it's your car mate.
Thanks for the reply, im sure your not that old .Surely the mod should be based on how useful your lights will become, or am I just showing my age?!! Having said that, black headlights will match your other black bits! I’d say give it a go and if you can, make it reversible so that you can go back to std if it looks naff. Give the lenses a polish up whilst you’re at it, you’ll be amazed at how good they’ll look standard.