After blocking Erron Black’s Boot Drop, or +, he has the option to Amplify it to send out a low gunshot. Amplifying the + will counter your attempts at attacking. If blocked, Erron Black will be at -10 but this is generally safe due to pushback.
This can be punished by jumping straight upwards and doing a jump attack after blocking the +. The low gunshot will miss and you are able to punish Erron Black with a full combo.
If you want to switch positions, you can also do a forward jump and punish from the other side.
Another way to punish Erron Black’s Amplified + is to do a Short Hop. By lightly tapping the Up button, you will hop over the low gunshot and can punish Erron Black upon landing. This is especially useful if you want to punish with a Krushing Blow such as Noob Saibot’s +,+,.