It won't 'darken' the paint as such, but it is 'colour charged' meaning it's a dark coloured cream. The othe PB's glaze is 'White Diamond' and that's a white liquid, obviously and aimed at light coloured cars. I have both these and the function of them is to mask and hide the small and even microscopic swirls and surface defects on the paint that collectively rob your finish of depth and gloss.

Because of the colour, they 'fill in' these defects and make them far less visible. In a way, it's a bit like an alternative to machine polishing where instead of removing a few microns thickness of paint to get rid, they just get filled in instead. It's nowhere near as effective, and obviously only a temporary measure, but on paint with a lot of light swirls and where machine polishing isn't available, adding a glaze after polishing and before waxing like either of the 2 Poorboys ones mentioned, can help the appearance. It's more effective on dark coloured cars where swirls etc. are much more visible.

Also, like most glazes they add oils into the paintwork that might have dried out over time if the car hasn't been cared for.