As you mentioned, the density and competition for direct sunlight is causing some of your blue spruce trees to physiologically shed their needles along the density- and light-affected regions of the tree (blue spruce is considered intermediate in shade tolerance which is why these trees are exhibiting signs of branch die-back--assuming no other biological factors are affecting these spruce trees).
In order to alleviate some of these density and light effects, pruning and or thinning may be required. Pruning and thinning operations, as you mentioned, should typically occur during the dormant season to negate the biologically active effects of growing season pests and pathogens.
In order to assist you with this effort, might I suggest you contact a local PADCNR forester for an on-site assessment.
Best of luck!