(BB 2015 & Real Band build 417. Win 8))
As I understand the manual to merge I highlight my four tracks (mono), select destination track in menu box, decide to keep fx etc. However on OK I am told my destination track has data on it and therefore cannot be used. It hasn't. It does have straight blue lines though.
In fact all 48 tracks are seen as having data on them as I cannot use 'insert blank track command either'

Am I configuring 'new file' incorrectly in some way?

sounds odd but i think - and i'm not at my music pc at the moment - a blue line is showing data of some form but not anything that will play.

if you select the track you want to merge to and use the edit/track/erase data does the blue line disappear and allow you to merge the tracks?

if so that's a workaround but why blank tracks have blue lines on i'm not sure.

Well 2015 is pretty old, the menu paths and options would be different than more current versions

If you Hover the mouse over the Track icon, a popup should appear, in that popup, does the Events item show a number?
If it is greater than zero, the track has data.
As mentioned; erase the data and try again. Or, if comfortable simply 'Erase Track' (which should give you a new blank track).

Or maybe try merging those tracks to a Wav file, and then importing the Wav file to a blank track, just a thought as a workaround. Like I said 2015 is pretty old and I don't recall this issue, maybe because I am pretty old too.