You can colour epoxy with poster paint.
You know, the powdered paint that they mix with water to change the colour of play group children....I mean for the kiddies to paint with.
Just mix a little of the powder in when you mix the two epoxy parts together....China restorers use that technique to mend broken pots...Obviously getting a perfect colour match is a skill, but black should be easy.
Epoxy is best cleaned up before it sets, with water...Warm water is best, maybe with a (really tiny) drop of detergent. Solvents are no good.
Epoxy is actually not brilliant at handling heat, so is not ideal for engine repairs.
There is a one part polysulphide glazing sealant called Arbocol. which is trowel-able/workable, and would fill cracks ect.
It dries like a hard rubber and comes in various colours (probably black).
Later this week I will be using a butyl rubber glazing medium for the first time....I will report back if it appears useful for this job.