On each isochrone, a blue dot indicates the point closest to the finish on that isochrone. This is very useful to place a pivot point wisely. In this case, we'll give priority to ranking up to this isochrone.
In example, if the best routing is very far from the direct route, it is recommended to make a second routing with a pivot point passing through one of these blue points. If the second routing is very close in time to the first, it's often a good idea to follow it.
Read in the blog Ces isochrones...