Backup recovery with dd on SM-G800F
According to files in the /dev/block/platform/*/by-name directory, my Recovery partition is in fact /dev/block/mmcblk0p10. The ending of the name of mmcblk0pX matches all the identifiers on the PIT, so thus I conclude that the identifier and the mmcblk name does match.

The command now will be dd if=/dev/block/mmcblk0p10 of=/storage/sdcard1/twrp.img bs=4096