I was told about this "rule" over 6 years ago
it's not new. It's older than the "It's no longer a blackboard, it's a chalkboard" thing.
( Jenk Neptune is a pedant and now I can't fit my sig in, Fri 12 Jan 2007, 13:34, archived)

Ummm ... no it isn't.
The blackboard/chalkboard legend was in the daily mail when I was still at high school some time in the mid-to-late eighties.

And this 'rule' isn't a 'rule' ... it's bollocks.
( Dr. Shambolic je suis charlie, Fri 12 Jan 2007, 13:39, archived)