Shouldn't a UV LED fan be clear?

Marketing - it's a hell of a drug.

Ok, but seriously, it's almost definitely not actually UV like a tanning bed. Most of the time they limit the bandwidth very specifically (forgot the range) so it's actually only just barely in the UV range, mostly it's purple.

As for the black... couldn't say. I'd have to see the fan I guess. The fan housing and the fan itself are very different... probably the housing is just plain black plastic and the fan blades are UV reactive.
 
I don't know yet tbh, I'm still waiting on my T-virus reservoirs to arrive so I'll know soon I hope :p
but this is the stuff I have
http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=59_413_992_995&products_id=30613&zenid=0edf40b0ceec602705da2304bd46baf2 I'm U.K. so ignore price
heres the images from that site though, third one is the uv effect
http://www.performance-pcs.com/catalog/index.php?main_page=popup_image_scroll&products_id=30613&image_scroll_id=0

I didn't go for the Uv on purpose though, saw black hose on ebay for a fiver and thought they fit my build better than clear hose, the Uv from the T-virus's cathodes wont be very effective on my build though as I'm a cable management freak, and that includes water lines hehe :p
Moto
 
Nice! I bought blue hose to go with all the fan LEDs in my Antec 902. It's also UV reactive, but I didn't like the UV light at all. I though it might make parts of the mobo glow as well, but it didn't so it was kind of lame haha. I don't use the UV light at all, I'd rather just light it up with LEDs - though I haven't got around to it yet. The top fan seems to give off enough light anyway :D