Hi John ,
In Mendix, every if statement must end with an else statement.
So since you are using 2 if statements, you need to use 2 else statements as well.
As far as I know there is no "Do nothing" option. I can't say my suggestion is correct, but you can use something like this to solve your problem.
Instead of do nothing, you can use something like " 1=1" for example. I hope it works for you .I would be glad if you let me know after you try it.
“if $Account/Blocked != $Account_2/Blocked then
if $Account/Blocked = true then $Account/BlockedSince = [%CurrentDateTime%]
else $Account/BlockedSince = empty