Are buried footings required for a shed or other structure to withstand 90mph winds?
To resist wind forces, you don't necessarily need footings below frost line, but you do need a substantial mass to resist the overturning forces from wind. Buried footings work well because the the wind must also pick up a substantial amount of earth in addition to the foundation weight. The deeper the footing, the more earth being picked up, so it is a good solution that does not require excessive amounts of concrete or masonry to provide dead weight.

The structural portion of the IBC specifies how much pressure 90 mph wind exerts on various surfaces. While too complex to get into here, if you understand overturning moments, determining the required dead weight to resist the wind is not all that complicated.

There are too many unknowns to say for sure, but I doubt a course of blocks, even with the cells filled, will be adequate to resist wind forces. In order to not build more foundation than you need to, it's likely worth it to find someone who can calculate the minimum foundation to do the job. An engineer is an obvious choice, but since no permit is required, it's likely a licensed engineer is not required. You just need someone who understands the physics involved and can do the math.