I am looking for a black sealant to help water proof/rust proof my car. There are some crevices underneath and gaps that need filling. Some upward facing pockets that could fill with water etc. A Caulking gun solution would be preferable and it needs to be available in black. I don't want it so set absolutely rock hard and stick like sikaflex. I bought my car part built and getting the aluminium panels off that had been stuck on with sikaflex was PITA.

Any ideas?

how about
http://www.screwfix.com/p/no-nonsense-roof-gutter-sealant-black-310ml/67521

this months CKC magazine had some chat about using waxoyl and how to apply it.. as rust prevention rather than filler - so may be off a little from what you are trying to do, but just thought I would say.

2 x PU18 High Strength Polyurethane adhesive sealant AUTO MARINE POND

this stuff is cheap for a pu sealant good grip over paintable etc and they do it in black

If its for sealing and not as adhesive (so not PU) then plain old silicone sealant. Available in black and tubes to fit the gun.

Alternative is "brushable seam sealer" Available from good factors, tougher than silicone but grey and in a tin.