TECH QUESTION: Esthetically Pleasing Material to Fill in Crack on Plastic Oboe? Please help.

I'd use an epoxy glue - and as Gordon suggests, I'd avoid the fast-setting stuff.

You can use acrylic paint to stain epoxy resin - just nip down to your local art shop and get a tube or pot of black.
You won't need much - a blob large enough to sit on the end of a matchstick should be enough to colour a small amount of glue...and you can always add more as you mix.
A little heat when you're mixing the glue and pigment will help, stick the mix under a desklamp for a few minutes. It'll temporarily thin the mix too, making it easier to flow into the crack - and it will cure faster.
It's worth slooshing the crack out with cigarette lighter fluid first, just to remove any grease that might have got in there - give it half an hour at room temperature to fully evaporate (or heat it with a desklamp).

Once you've applied the glue it wouldn't hurt to gently warm the area for a little while, to help the glue bond.
I also often use a hot needle which I poke into the joint. This helps to thin the glue and remove any trapped air pockets. You'll need a flame to heat the needle, and it's important not to get it too hot or it will burn the glue. To save on cleaning up later you can use cigarette lighter fluid to clean off any excess glue from around the joint.

Rather than waiting for the glue to dry before trimming it down, you can slice it with a sharp blade when the glue is about 3/4 cured. Keep a blob of the leftover glue handy so that you can test it from time to time to see how cured it is.
It takes a practised hand though to avoid digging the blade into the joint, so rather than go for a 'flush' cut you might just want to use this method to reduce the amount of dried glue you'll have to file off later.

Regards,

Gordon (NZ) said:

Stephen, are you saying that acrylic paint, witih its water base, mixes with epoxy? In my experience water will never remove epoxy from my fingers, so I intuitively think epoxy would not mix with water based paint.

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I wouldn't have considered it either, but I got the tip from one of the boffins at Araldite. I'd been buying their tubes of pigment but wasn't able to get quite the range of browns I needed (their own brown is massively biased to the red side) even by mixing the pigments - and I was advised to try acrylic paint.
I thought at the time that it was jolly decent of him to tell me, considering they sold proprietory pigments, but I think it was clear that I was never going to get through more than a handful of their tubes in a lifetime.
I found a little heat was necessary to get a good mix - but I always heat epoxy anyway to ensure consistency - and I reckon it gives about a 10% retardation factor in curing times, though as a fringe benefit it seem to extend the 'elastic' time, which makes slicing the excess away a lot simpler.

I daresay it's not a method that's of much use if you tend to only need to stain the glue black, and you have ready supply of spirit stain pigment - but if you want to 'go brown' it's pretty much the only way to get a decent colour match.

Regards,