A Bb5 is a major version of what is called a double-diminshed chord. A double-diminished chord is a stack of minor thirds Bb-Db and Db-Fb (aka Bb-Db-Fb).

Make it major and you get Bb-D-Fb.

It's pretty rare put that what it is.

I agree with John. There is usually a 7th involved and the key does not always serve as an indicator.

I love Diki's idea of a passing chord. It ensures that you never play a wrong note. I was getting to the right note, by passing through all of these other ones.