Coding Problem

Ok, crashcourse on indenting:

In Blitz there are single commands, and commands that define a scope. Examples of a single command are Print, End, Plot, Rect, Line, LoadImage etc.
Examples of scope commands are:
For-Next
If(-Else)-Endif
Select-End Select
Function-End Function

etc.

Key rule: you should indent the scope/area of each scope-command. As an extra bonus you might want to have an empty line around scopes.

Based on personal taste: I also indent drawing/actions on a canvas or image between changing the drawing buffer and the flipping, but that's personal.

One might even indent creating menuitems in a menu.

As long as things belong together for a limited area in your code, you may indent them.

So, to apply this on your function:

Function StartEruP()

  MainMenu=CreateWindow("Main Menu",300,300,400,500,main,15)

  Ev=WaitEvent()
  Es=EventSource()

  Repeat

    Select Es

      Case MainMenu

        If Ev=$803 Then

          Blah=Proceed("Do you really want to quit?")

          If Blah=1 Then
            End 
          Else
            If Blah<>1 Then
              Notify("Find out what you want next time")
            (EndIf ; according to b32)
          EndIf

        EndIf

    End Select 

  Until KeyDown(1)

  End 

End Function



StartERuP()
End 

and with dots to show how an end-scope matches a begin-scope:

Function StartEruP()
.
. MainMenu=CreateWindow("Main Menu",300,300,400,500,main,15)
.
. Ev=WaitEvent()
. Es=EventSource()
.
. Repeat
. :
. : Select Es
. : .
. : . Case MainMenu
. : .
. : .   If Ev=$803 Then
. : .   :
. : .   : Blah=Proceed("Do you really want to quit?")
. : .   : 
. : .   : If Blah=1 Then
. : .   : . End 
. : .   : Else
. : .   : . If Blah<>1 Then
. : .   : . : Notify("Find out what you want next time")
. : .   : . (EndIf ; according to b32)
. : .   : EndIf
. : .   :
. : .   EndIf
. : .
. : End Select 
. :
. Until KeyDown(1)
.
. End 
.
End Function



StartERuP()
End 

Note that the Case command defines a scope, but since it doesn't have an 'End Case' command, I couldn't really draw dots here, or at least: it would look a bit weird compared to the rest.

btw, while we're checking code anyway:

Function StartEruP()

  MainMenu=CreateWindow("Main Menu",300,300,400,500,main,15)

  Ev=WaitEvent()
  Es=EventSource()

  Repeat

    Select Es

      Case MainMenu

        If Ev=$803 Then

          If Proceed("Do you really want to quit?") Then End 
          Notify("Find out what you want next time")

        EndIf

    End Select 

  Until KeyDown(1)

  End 

End Function

Wiped a few lines. If your blah would've been 1 then it would've ended, if not it would notify, and you wouldn't need an 'Else' nor would you need to check on blah<>1 because it HAS to be <>1 in order to get at the line in the first place. (if it was 1, then the program would've ended!)
It's like a factory where they make blue and red paint. At one point/junction one sends blue paint to hall 1 and red paint to hall 2, would it be practical to have other employees standing in halls 1 & 2 to check whether the colors are correctly chosen, after it was already correctly re-routed at that first junction?