Why supply Taint it Black AND Dark Matter? Don't they do the same thing?
Not always, Dark Matter is a great dressing and quick to apply but it won't last as long, or recolour as well, as Taint it Black. If trim is really pale or faded, go for Taint it Black rather than Dark Matter.
Will Dark Matter attract dust or sling onto paint?
It shouldn't do if it is wiped 'dry' after application. Just let it sit for a minute or two and wipe away any residue before driving off.
What if it rains straight after application?
Not ideal. There could be some streaking, so you may need to reapply a second consistent finishing coat once things dry out.
Any seasonal advice?
Trim fades through UV damage, so ensure dark plastic trim is dressed or coated throughout Spring/Summer. And your wheels are going to be really filthy during the Winter, so give them a good scrub with Pneu Look after the worst of the bad weather.