That so-called 'blue plane' is NOT redundant when you're creating a Sketch while referencing anything other than an existing Datum Plane.
As for the comment about 'Auto Constraints' what it means is that the software has managed to constrain the Sketch using a combination of explicit (user applied) and auto (system applied) constraints. Now this does NOT means that the Sketch can't be modified by dragging around geometric objects, just that the system has found a constraint solution. The number indicates how many constraints the system had to apply to get that solution. Now it may be possible to fully constrain the Sketch with fewer explicit (user applied) constraints, just that this is one valid solution. Now you can convert those into explicit constraints if you want, or use them more as a way of finding what parts of the sketch ars not yet constrained and that perhaps the auto constraints can give you a hint as the type of constraints that could be used to fully constrain the sketch.