I'd still order one, these are amazing, and I don't really see/notice the discoloration, not in line of sight. That said, I have an email to Tackform about this.
I've definitely observed the same black --> brown (or almost gold) discoloration on other cheaper anodized aluminum items that were exposed to the sun.
Mine hasn't shown any signs of discoloring and my truck spends a lot of time outside (but I'm in Cleveland not Phoenix). To the best of my knowledge I have one of the really early ones they made for our trucks. I wonder if they got a bad batch of parts?