The priority is
1 - Among the First
2 - Near the Top
3 - Middle
4 - Bottom
Any blocks competing for a spot should appear in the order they were created within that spot.
If your custom block is the only "Among the First" block you've created, then it should appear directly after the search. If it appears before the search, then I think it's a bug. If the Share block is appearing first, you can try changing that block's position to "Near the Top".