From what I have been reading lately, I was under the impression that blackout days in summer for So Cal AP's were actually less crowded than days they could go. Hmmmm.....
In the last year or two, your conclusion has mostly been the case. (The largest exception I can think of was the two weeks after Christmas up through New Year's, which were incredibly busy despite being ultra-blockout dates when even Deluxe passholders couldn't get in.)
Last week, we found the park to seem more crowded on Wednesday (not a blockout day) and then oddly, Thursday (Grad Nite, blockout day).
Friday (blockout day AND World of Color's premiere) was not too bad for most of the day and Saturday (blockout day for even Deluxe passholders AND we arrived late!!!) was overall light; even a CM working said so, so it must be true.

Sunday (not a blockout day except for the cheapest APs) was expected to be very busy.
Weather can affect things too--if it looks like it might rain, locals do tend to stay away.
But I think the difference between Saturday and Sunday, at least, was due to blockout days. The wait board picture I have from about 11:30 AM Saturday is very similar to the one I have for about 10:40 AM on Sunday.
Here's Hydroguy's popular opinion on blockouts and how they affect crowds, which were accurate for this trip (again, late December was the only time that I heard blockout theories absolutely didn't hold true):
http://www.disboards.com/showthread.php?t=2365983