That's not quite true. A penalty is a foul within the box, regardless of whether or not it denied a goal scoring opportunity.
Thats the law of the game, you are right, but referees also work to "guidelines" (haphazardly) as to whether a penalty kick or sending off is awarded depending on the situation.
If we go
solely by the rules, which is what the pundits and the opostion fans wanted yesterday it
still isnt a penalty, observe the following from Fifa's rule book-
Direct Free Kick
A direct free kick is awarded to the opposing team if a player commits
any of the following seven offences in a manner considered by the
referee to be careless, reckless or using excessive force:
• kicks or attempts to kick an opponent
• trips or attempts to trip an opponent
• jumps at an opponent
• charges an opponent
• strikes or attempts to strike an opponent
• pushes an opponent
• tackles an opponent
A direct free kick is also awarded to the opposing team if a player
commits any of the following three offences:
• holds an opponent
• spits at an opponent
• handles the ball deliberately (except for the goalkeeper within his
own penalty area)
A direct free kick is taken from the place where the offence occurred
(see Law 13 – Position of Free Kick).
Penalty Kick
A penalty kick is awarded if any of the above ten offences is committed
by a player inside his own penalty area, irrespective of the position of
the ball, provided it is in play.
So seeing as Rafael neither acted with excessive force, tripped, held or acted in a careless manner, the very best Blackburn could have hoped for in that situation, was an indirect free kick.