[MENTION=202160]justagirl76[/MENTION] I've yet black out my tails on my 4runner (I just dont want too really). however I did it on my previous mazda.
First off I'd recommend for any mod taking the part off the car if possible. I've removed the taillights on my runner before to change bulbs, so its a simple process (just need a 10mm socket with extension if I remember correctly)
The first time I blacked out my tails I used VHT NiteShade, it comes in a spray can. While I liked the results at first, it began to crack down the road and would have required removing it and re-applying. This is probably the easiest way to do it if you don't mind having to re-apply down the road.
The second time I used Lamin-X film. This took a little more time/skill since I cut the film myself for my tail lights in particular. Still a fairly easy job though after doing online research, and IMO a much better and longer lasting solution.