Cumdumps love to take loads - it's their primary motivation for having sex. They will feel unfulfilled if the top doesn't cum inside them. Cumdumps also tend to be relatively submissive. They like to service the top and get him off.
There are two types of cumdumps - anal cumdumps and oral cumdumps (plus cumdumps who like both). An anal cumdump may give you a blowjob - they may not, but they don't want to suck you off - they want you to fuck them and cum in their ass. In contrast oral cumdumps will be eager to give you a blowjob - they're not looking to get fucked - it's all about oral sex for them.
Please don't be rough with the cumdump unless you know they like rough sex. Some cumdumps prefer you to be somewhat gentle - being rough with them could be misconstrued as rape.