Name of this kind of stain on fruits and how do they form
You never visited the avocado blemish identification fanpage? do you use google? it's quite good.

http://ipm.ucanr.edu/PMG/C008/m008fpfrtdmg.html
Causes of avocado Damage

Avocado thrips

Anthracnose

Caterpillars

Branch canker and dieback

Fruit and stem-end rots

Genetic mutations

Greenhouse thrips

Latania scale
Mechanical injury

Nutritional disorders

Off-season fruiting

Persea mite

Phytophthora fruit rot

Sooty mold

Sunblotch

Sunburn

Vertebrates

Weather extremes

Wildland fire

mosty they are insects, funghi, viruses, mechanical forces, and sometimes, like the woody avocado, the cause is unknown:

referenced.