Aerospace 303 Protectant is great to rejuvenate faded plastics. It's more of an upkeep sort of thing than an outright solution but if you do it every week or two for a couple of months it'll hold that deep black for quite a while. On my Miatas and have used it on my door seals, dashboard, convertible top, etc. I think they've expanded their product line a bit now but I haven't checked in years.
I also have shin etsu grease which works wonders on window tracks and the sort. I'd probably suggest that for heavy duty weather seal treatment. That shit is amazing.