I think the success of April 1st was due to the programming block's packaging AND the content of the programming itself. It wasn't just that people were excited to see TOM back in action reviewing games and taking names, it was that a whole night of surprise programming came with it as well. The shows themselves have the biggest effect as can be seen from the change in ratings from show to show.
It certainly will lend credence to the theory that blocks and their packaging can have an affect on people's viewing habits if done correctly.
I think this is the key. Toonami wasn't just successful because of packaging done correctly, it owed just as much success to lineups done correctly.
People often complain about the lack of promotion for Adult Swim Action. Quality packaging helps with that since a block pretty much is promotion and branding. It says, if you like X type of shows you will see them during this time period and said time period will be designated by Y packaging.
For a block to work the programming has to live up to the promises of the branding, and the branding has to fit the programming. If a block can establish itself as credible on both fronts it can work wonders to get people to continue tuning in and to check out future shows.
Adult Swim Action seems to have lost most any similarity to a true block at this point. The shows are there, but there is nothing to unify them. When ASA started out it didn't shy away from the action aspect and had Toonami-style clips from the shows going in and out of commercial breaks, etc. Sometime after those were dropped, they at least had the still images from Japan during those hours.
Now it could be any hour of Adult Swim. When, say, Durarara started on Saturday nights, it had some ads, but nothing to really build off of. That's not to say viewers couldn't find it and like it, but it was lacking certain advantages.
Take Toonami for instance. Toonami was able to sell its audience on the idea that shows worth watching aired within the block. Thus, when say, Outlaw Star, debuted, there were not only the ads and simply people finding and watching it, but also people thinking, "Toonami gave me Gundam Wing and Tenchi. Those were awesome. Maybe I should check out this next new show they are airing..."
The thing is, none of that works, no matter how awesome the packaging is, if the shows don't live up to the promise of the packaging. Thus, a block is just as much about the content of the programming within as it is about the packaging.