Giving a blow job might be in a submissives chores in order to give pleasure to their Dominant. Some submissive might have a really hard time to swallow the cum. Some claims that it is an acquired taste and I tend to agree with that as you can learn to swallow through training.
I would recommend anyone that is doing training in this area to take a look at the site Don’t Spit, Swallow Cum website. They have a lot of useful tips on how to make it easier to swallow, how to affect the texture and the taste of the cum. I would even say that it is required reading for ever aspiring cum slut.