You can also purchase SunGuard ( you can find this at sunguardsunprotection.com) and add this to your laundry during wash cycle to add UPF to any of your clothing.
Black pigment as far as I know absorbs the the rays of the visible light spectrum however UV is in the invisible part of the light spectrum. I think you can probably find UV light absorbent t-shirt if you find a factory that uses UV absorbing pigments.
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I just read this in a mag.
Thanks for that information :-) I have been looking for clothes with a high uv-protection that aren't sports clothes for a long time...
Here is a link
http://pcp.oxfordjournals.org/cgi/content/abstract/10/2/325