Maybe have it as a swipe-in gesture from the left/right edges (for right/left indentation, respectively), with real-time visual feedback (highlighting the affected Block) and option to cancel by swiping back off screen (in case user starts moving the wrong Block - which might happen when dealing with vertically short one-line Blocks).
After the 0.4.0 update for Android, indenting blocks has been made a bit easier by dragging a block under or on top of another block. However, you can’t indent the current block that you’re editing without selecting another block, long-pressing on it, then dragging it to the desired indentation, as long-pressing just brings up the text style editor.
A button, or as qualquertipo mentioned, a gesture to indent and outdent on mobile is a feature which is essential for a block-style editor. Most apps (Notion, Craft) which I am familiar with on mobile devices use a button, but a gesture may also work well.
A direct approach like it is available in logseq, or how apple notes does that swiping the bullet o the right or to the left not just moving. Moving would work great when multiple items be indented or outdent.
Dedicated indent/ outdent is quicker and intuitive.
Or like Apple notes way.
Maybe title can be changed to emphasize that clearly.
Dedicated indent/outdent quick action or button