Instead it would be:

  if ((input = 'y') or (input = 'Y')) then
    writeln ('blah blah')

  else if ((input = 'n') or (input = 'N')) then
    writeln ('blah')

  else
    writeln ('Input invalid!');

  case uppercase(input) of 
    'Y': writeln('blah blah');
    'N': writeln('blah');
  else
    writeln('Input invalid!');

  if (input = 'y') or (input = 'Y') then
  begin
    writeln ('blah blah');
  end
  else if (input = 'n') or (input = 'N') then
  begin
    writeln ('blah');
  end
  else
  begin
    writeln ('Input invalid!');
  end;

And why should be the double pipe be any better to read than an "or" statement?

Btw: as you won't need the begin and ends it would look like

  if (input = 'y') or (input = 'Y') then
    writeln ('blah blah')
  else 
  if (input = 'n') or (input = 'N') then
    writeln ('blah')
  else
    writeln ('Input invalid!');

  if (input in ['Y', 'y']) then
    WriteLn('blah blah')
  else if (input in ['N', 'n']) then
    WriteLn('blah')
  else
    WriteLn('Input invalid');

  if (LowerCase(input) = 'y') then
    WriteLn('blah blah')
  else if (LowerCase(input) = 'n') then
    WriteLn('blah')
  else
    WriteLn('Input invalid');