I doubt it'll affect the parks. DTD maybe, but I can't see thousands of people who have already had a jam-packed day going to the parks afterwards. The day before and after may see Blizzcon attendees trickling in. It depends where people are coming from, they may do Blizzcon and the parks in one trip if they're coming from farther away, but the days OF Blizzcon, I don't think you'll see any rise in people. You may even see a decline.
If I were going that weekend I'd be more concerned about hotel prices and making sure to make reservations for any out-of-park meals in advance, since hotels and restaurants are going to be packed.