Here's a message that I e-mailed to a buddy with a Blackjack a month ago when I measured the brass adapter sleeve:

Brass adapter sleeve in stock prop CAN be pushed out and placed into a 3/16" CF series prop. Fits perfectly. Brass sleeve is 3.0mm ID, 4.75mm OD. CF45 is on and perfect!!

I didn't measure the length of the tube however. I had several of the K&S Engineering "Odds and ends" tube assortments that I bought from Hobby Lobby. I wne through them until I found a tube that was the same dinensions, then used a small 2" Harbor Freight metal cut off saw to make some more of the adapters about the same size. They fit snug in a prop and don't require any glue.

There's an insert with the BJ's instruction manual that states that they changed the prop from what was originally used. My guess is that they had to use what prop was available, but their shaft was already too small, so they came up with the adapter to solve the problem.