The problem with a global priority is that it will cause all the blocks to have the same priorities.This was requested couple of times, but the problem here is any custom ordering can't be automated. If you arrange some ads manually, it prevents ads scheduler from creating the blocks in fully automatic mode. What happens with the order when some ads are expired, new ads added? What if more than 9 priorities are added, what do you think?
The same commercial that in a block is the first, could be the last in another.
With this approach it is not possible.
In addition to the commercial priority, we could think of a commercial priority per block. (By default at 5 and it is adjusted by moving the ads)
Something like this
It's not a brilliant idea, but it's what I can think of at the moment.
Another solution is to migrate priorities to the order of this list, but the option of a global setting for all blocks is lost.