To use _block_id inside a perch:repeater, just do this:
Add scope-parent to the perch:repeater, then add id="parent._block_id" to the ID inside the perch:repeater
When combined with perch_item_index, this will output a unique value per item inside a repeater. A useful addition if you intend to have more than one block on a page.
More info in the docs, at the bottom of the page, 'Accessing content outside the repeater'.