A block is when you knock the ball out of the air or your opponents hand while they are shooting.
You can get this naturally while upgrading your player no mater what position you play., but obviously it will be easier to get if you are a big man in the forward or center position. If you want to try and get this quickly without your main character, you can create a big man who is 7`3`and upgrade the stats as much as you can for blocks and you might as well do offensive rebounds at the same time. Give your guy as much wing span, height, etc... as you feel will be the best combo. You can also try a smaller guy at like 6`11`with more vertical and speed, yet I felt the traditional giant the easiest to knock this out quickest, since my main character was a PG
Then camp out near the rim, making sure to pop in and out of the key so you don`t get a 3 second violation. There is balance of defense you should try to maximize your chances. Play defense on your man to block other bigs but be ready for guards driving the lane or doing pick and rolls so you can protect the rim when they come for dunks or layups. Time your jumps to block the shots with
Play on longer quarters and easier difficulty to maximize your playing time and chances to get these achievements.