You need 700 boxes for about 50% chance, 1400 boxes for about 75% chance and 3000 boxes for about 95% chance
ETA: People generally assume that if something is 1/1000 then you need to do it 1000 times to be "sure". If you open 1000 boxes you have only about 63% chance of getting a Blemmigan
edited by genesis on 3/4/2018