What do you understand by blended axis?

A blended axis puts two or more measures on a single shared axis rather than on two independent ones.

You create it by dragging the second measure onto the axis of the first until the two green bars appear, which brings in the Measure Names and Measure Values fields.

Because both measures are read against the same scale, the comparison is honest — which is exactly when to use it: measures in the same unit and of similar magnitude, such as this year against last year.

This is the counterpart to a dual axis. A dual axis gives each measure its own scale, which is necessary for different units but makes it easy to imply a relationship that is not there. If the units match, prefer a blended axis or synchronise the dual axes.