How does a nerve block help endometriosis?

Only the pain. Nerve blocks are better for defined areas of the body. Nerve sensation from endometriosis is more complex and less amenable to this treatment as a long term.
May to a degree. Nerve blocks can be given any many locations by a trained physician to decrease pain from sensory nerves endometriosis pain can be in multiple locations so you need to determine most severe area you wish to treat bu it may not completely relieve all areas. More important is to seek your gyn doctor's help to treat endometriosis pain directly.