I think this behavior came about because at first it wasn't possible to edit a blue Constraint at all; the user first had to change it to a regular (driving, red) Constraint and then change the numerical value. After blue Constraints were introduced there was a discussion to the effect that when someone wanted to edit the numerical value of a blue Constraint, the user therefore wanted that Constraint to be a driving (red) Constraint. So, as a time-saver, editing the number value of a blue Constraint caused that Constraint to become a driving (red) Constraint.
I think the bug is that editing the Name of a blue Constraint, so that it can be used in a Spreadsheet / Expression, also changes the Constraint to red. Instead, a blue Constraint should only become red if the Length / Angle field is edited.
This recollection is all from memory; I think Abdullah or maybe Normand will be able to provide more input.