"My question is, how do you people, practising an internal art, train to defend yourselves against a blade?"
My old taiji class (Yang Lu Chan via student of Erle) trained with rubber knives and such. We used our taiji to defend. You could disarm someone with a knife with a good internally powered wingblock (once did it to my instructor).
Once you are moving internally and could generate some power (explosive power - fajing - in particular) - I would say just play with someone attacking you with a rubber knife. You learn a lot that way.
I don't think enough of this has been explored (or known) by most taiji people - but with my own experience in some of this kind of training, I know the defenses are there.
~ Eric Putkonen
(Teaching Tai Chi Chuan in Plymouth, Minnesota)