I frequently expect my boys to 'clean up after themselves'... but I actually enjoy giving blowjobs as occasional rewards or just because I'm feeling like spoiling one of them at the moment, too. Cum's a naturally produced substance - I expect them to taste my juices on occasion, therefor think it's quite fair that I taste theirs, that they taste theirs, and certainly on occasion that I taste my own. ^.^ It's a hell of a lot cleaner than most other body fluids, it's easy to adjust the taste to sweeten it up and make it more palatable, and it's a very personal thing.
You might want to spend some time thinking about ~why~ the idea bothers you: is it the taste? The texture? Is it the level of humiliation? Perhaps you would find it easier (assuming your Mistress would allow this) to lick it out of her, tasting her first, making the act a bit more sexual. Lots of posters have already given suggestions for altering the taste - the website someone linked has a great smoothie recipe that should work nicely - and if it's about the texture, start small. Ask her to feed you a few drops on her fingers, licking it off; imagine it's honey she's spilled on her hand, or a bit of ice cream, or even her own pussy juices that you're getting to lick up. The mental image may help you get past the first moment of distaste.
Above all, remember that you're doing this ~for her~, yes? Watch her face. Seeing her eyes light up when a drop or two touches your tongue may be enough to help you find a mental point where you can be, if not comfortable with the idea, at least able to cope with it long enough to really make her day.