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How strong are the winds, what MPH gusts do you get?
There are different methods for varying MPH, the higher the MPH the more that must be done to secure a structure. In a windy area face the corner of the building into the wind. This will reduce the surface area that the wind can push against.
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A pressure treated bottom plate is only required if the walls will be resting on concrete that has ground contact. Since you're building a frame that will have joists with plywood on top of that, your walls will sit on this as a base, a PT bottom plate isn't a requirement. Your frame and joists should be built using pressure treated lumber, though not a requirement for a shed.
IBC states that any shed or playhouse 120 square feet or less does not require a permit, some towns or cities may have code with more stringent requirements than the IBC standards.
If anchoring is not needed, then in each location of the concrete blocks, dig out a 24" wide x 24" long x 12" deep hole, fill with gravel. This will provide a stable surface to place those blocks onto and prevent them from shifting or sinking.
If anchoring is needed, dig an 8" round hole to the frost-line, fill with concrete, place anchor bolts into drying concrete. (Make sure concrete is firm before placing the anchor bolts or they will become concrete reinforcing bolts instead.
) This is a lot of work for a shed, but a requirement in some municipalities.
Alternate method: Level your existing blocks, then use Ground Anchor Screws a.k.a. Tent Anchor Screws. Some are rated at 4 tons per Anchor Screw. The advantage of these are they can be added later and attached to the frame with galvanized wire.
To level for an 8'x8' frame, you will need a 10' 2x4 with no crown that is straight and a 4' level. Standing the 2x4 on it's short side with level on top of that, you now have a 10' level. Place one end of your 10' level on any block, the other end on any one of the other three blocks. Check for level and make a note of the height difference, the space between 2x4 and block, with a ruler. Check the other 2 blocks in the same manner. Determine which of the four blocks is the lowest. Dig out the other three, checking often, until each matches the lowest block within 1/4".
To square, measure diagonally from one corner to another, note the length. Do the same for the other corner, forming an X. Each line of the X, / and \\, should be of identical length.
Edit: Line levels are OK on short distances with a very taught line, but on long distances the weight of the line will make the line level useless and inaccurate. Distances over 25' can have a lot of sag, but distances less than 25' should be OK. The greater the distance the more sag is present.
