Hi Jessica,
When you participate in a Twitter pitch party, you are not actually "submitting" to anyone. You are just posting your pitch for your book on the specific date to the feed for the party. If an agent or editor "hearts" your pitch, it means that they are interested in seeing the story or the first 5 pages, or whatever their guidelines state. That is when you need to do your research to see if that editor or agent is someone you think would be a good fit for you and your story. If not, you should not submit the story. No explanations needed. If you do think it's a good fit, you would follow the guidelines they outline for your submission.
As far as online blog contests are concerned, they are not the same as a Twitter contest. I do know Query Kombat is legitimate, and I know there are others who have posted success stories, but I am drawing a blank on them now.
I hope this makes sense!