kiwlm said:But what about the point when the bass line goes to play Bb, if I were to harmonize to that, is that F or F#?
Or am I totally getting this wrong?
I think you are getting something wrong - you can't harmonise every note of a bass line like this - we're not talking Bach Chorales here!!
As people have said - a passing note is not part of the key we are in - it's has no relation to any key and is just a "device" to make lines flow.