GTI, with a black car that is going to be out in the hot sun all day long, why not use both a polymer sealant like Klasse SG and then top it with Blitz? This way, you have no doubts as to whether or not your protectant is doing anything against the sun's UV rays. I currently use a glaze + Blitz on my black car because I feel it looks better than the polymer sealant + Blitz combo, but it's always garaged whenever not driven. If the car was ever going to be regularly outside for more than a couple hours at a time, I wouldn't even consider using my current regimen.