Long-pressing on the inserter button ((+) on the quick toolbar) should show options to Add Block Before and Add Block After the currently selected block. If there is no selection these options could be Add to beginning and Add to end. On iOS, this could be an ActionSheet and Android a BottomSheet.

Example:

Note: I think we could expand on this pattern in the future, but I've found myself wanting to do add blocks before the selected one quite often, especially now that we have InnerBlocks.

We might also want to add these options to the ••• menu on the inline toolbar when we get a chance, similar to web.