Oon October 1, 1991, U.S. Patent No. 5,052,418 issued to David Miller on a Solar Body Tattoo:

Miller’s method was to apply a template having a central opening to the subject’s skin. Sun block is then applied over the central opening, and the template is removed. Thereafter, the subject’s skin is exposed to tanning rays, and the sun block washed away revealing a skin tattoo.

Alas, Miller was not the first to use the sun for tattooing. See, for example. U.S. Patent No. 2,851,805, which issued September 16, 1958, on a Sun Tattoo Form:

It seems no matter what someone comes up with, there is always prior art to consider.