When editing a post from mysqlman, I click the Null checkbox for those fields I want to be left blank, but it's not saving my changes.
You have to delete everything from the column when you select it as null otherwise it won't change.
Column_Name [ Existing Content ] [x]
If you just hit X to make it null (X is just my version of a tickbox :) ) then you will still see Existing Content in the column.....so you need to erase it first (or at the time of checking the tickbox) eg:
Column_Name [ ] [X]
...then it will become null.