The presence of blue, as opposed to black or red, means that the line would actually go off of the current screen or above/below the currently visible measuring parameters. For example, 60-80 BPM on the graph.
This measuring feature is designed to enable you to see the parameter curve better.
So where you see blue, the line actually goes above or below the current graph, but is kept from “jumping” and thereby altering the current view too severely.
This allows for greater ease when viewing the HRV graph in general.