Weird, I'm working on the same thing (just started) and I'm having trouble with collision at offset too. I can get it to work but only if I put a 1 pixel space between the blocks, as soon as they touch it all goes belly up.
Here's the cap I'm testing with to show you better. Ignore the two blocks to the left and right, like I say, it's a test capx. The main bit are the sprites (can't use collision with tiles :( ) the number is generated depending on what blocks surround the black one in the middle. One above add 1, to the right add 2, beneath add 4 and to the left add 8.
If you move the blocks so they're flush against each other the number isn't calculated correctly. Anyone got any idea what I'm doing wrong?
Cheers.
CapX file