Yep, it's the result of an IK/FK blend.
There are actually 3 joint chains:
- brown/maroon for IK
- dark blue for FK
- purple for the resulting blend of the IK and FK

Just to elaborate on this:
It's usefull when u want to overextend a joint what the IK handle won't allow (e.g. for an arm or leg).
You'd better use Animate>Ik/Fk Keys>Set Ik/FK key on the IK handle (never on the bones!!) to animate your blends and FK. Both Ik handle and the joints need to be keyframed or else freaky things start to happen with your IK solver.

Steps to blend from IK to FK:
1. Set IK/FK Key on the IK handle when Ik Blend in Channel box is still 1
2. move timeslider
3. set Ik Blend in Channel box to 0
4. Rotate joint
5. Set IK/FK Key on the IK handle

Do your FK animation using Set IK/FK Key on the IK handle

Steps to blend from FK to IK:
1. Set IK/FK Key on the IK handle when Ik Blend in Channel box is still 0
2. move timeslider
3. set Ik Blend in Channel box to 1
4. Rotate joint
5. Set IK/FK Key on the IK handle

Before and after your FK animation, you could just use regular Set Key to animate. Though this is when u will see a dark blue FK joint chain hanging from the top joint of your IK.

Attached is an illustration from the Maya manual.
Using Maya>Animation and Character Setup>Character Setup>Skeletons>Blend IK and FK animation