Some of these Blues are not like the other Blues I'm sorting through used Lego bricks and found that in some stacks of blue bricks there are slightly different shades.  I can't find a common shade of blue that matches and I'm not certain that it's not just fading or discoloration due to time and exposure.  However there seem to be only 2-3 distinct shades which makes me think they're actually different colors.

Can anyone help identify what color they might not be if not Blue?

For comparison, I've put some of these blue bricks next to my color board of common blueish colors: from left to right Bright Light Blue, Sand Blue, Medium Blue, Medium Azure, Dark Azure, Blue, and Dark Blue



Note the one with the arrow seems very similar to the one 2 to the right of it and is duller &amp; darker than the reference Blue piece, though not as dark as Dark Blue.
It looks like these darker are the same bricks in Blue that have simply "yellowed" over time, which changes the shade of the color. Another answer explains the reason behind this process.