Blueberry, pain, fatigue, and depression signal different things. Pain signals local injury, fatigue signals not enough ATP in your muscles to do what ever it is you are trying to get your muscles to do, depression signals more of a global energy crisis, particularly in the brain. These things are to limit physical activity to reduce the damage that the physical activity will cause.

The “energy crisis” of depression is in the brain, but the current experience of depression is more of a problem with malfunctioning of the control pathways that regulate brain ATP levels. Low ATP in the brain is associated with depression. There are some other examples, vascular depression which is common in the elderly, there is reduction in ATP levels in the brain coincident with reductions in brain perfusion. Fix the perfusion and you fix the depression (and vice-versa because they are coupled). Bipolar disorder has similar low ATP status. ATP status is extremely well regulated. Depression and allied mental disorders are disorders of that regulation (which is very poorly understood).