Rigging - Why is my bone blue?

Question

I selected a bone and then tried to add another bone to the selection. But instead of the other bone getting selected, the first bone gets blue. I had to click another time on the second bone so I could actually select it.

What does the blue color indicate?

I made three clicks, I will post an image for each click, the red X indicates the bone where I clicked at.

First click

Now its blue... and the second bone is not selected

Last Click (why I had to click twice on that bone to select it?) The second bone is a duplicate btw, there are two exact bones at the same place.

Answer 1

The second bone is a duplicate. That means that your second click is actually selecting the duplicate bone. Your first bone turns a darker shade of blue indicating that it is selected, but not the active bone. The active selected bone is hidden beneath the one you want to select. When you click a third time you finally select the bone you want but will have selected three bones including the hidden duplicate. You can see the bones selected in the outliner window. You can select from there or hide the duplicate bone (select and hit "h")

Answer 2

In Pose mode. the lighter blue color indicates the active bone, while the darker blue indicates a selected bone.