Sounds great, except that I would space the blocks a bit closer together. Two possibilities:
4 10' 2x6, set ~ 2' apart (1', 3', 5', 7' relative to the 8' side) with blocks at 1', 5', 9'. That would give support for the crosspieces every 2' with the 2x6 spanning 4'. Total of 12 blocks.
5 10' 2x6, set ~ 2' apart (1', 3', 5', 7' relative to the 8' side) with blocks at 0.5', 3.5', 6.5', 9.5'. That would give support for the crosspieces every 2' with the 2x6 spanning 3'. Total of 15 blocks.
The 8' crosspieces could be placed 16" on center. Decking would go on top, your shed on top of the decking.
Keep in mind also that while one option is to get all the blocks level on the ground/gravel, the other option is to use short pieces in the middle of each block (size depends on the exact design of the blocks you use) that you trim to make essentially a grid of little posts-in-blocks that have the tops level. That's what I did for my deck - a lot easier than digging down.
You may also want to consider making the deck slightly bigger in one dimension so that you have space to stand on the deck in front of the door.