Dude,
if you like it normally its done with black spray paint. Clean the lens off as best you can, mask off the parts you dont want black (masking tape =) works great...you can razor cut it straight or curved as you like...). Then just blast the rest of it with black stuff.

This is not an easily reversed-process though, paint thinner might cloud or mess up the plastic surface if you try to remove the paint. Sanding will do the same. Just be careful or buy some turn signals from the junkyard to experiment on =)

Hope this helps.
-John

__________________
2009 Kia Sedona
2009 Honda Odyssey EX-L
12006 Jetta Pumpe Duse
(insert Mercedes here)

Husband, Father, sometimes friend =)