My guess was going to be that the mortar is more porous than the cinder blocks so there was more water infiltration coming in those areas and affecting the color regardless of whether the discoloration was done during or after painting.
However, upon closer inspection I can see that certain blocks are fully affected by the discoloration so that leads me to believe some sort of sealant was used prior to painting.
Most discoloration seems confined to the mortar and surrounding areas which is akin to the application results of grout sealant.
In particular, the left block seems fully affected but the right block was only half affected so the previous person just applied it more liberally than other spots for whatever reason.
It's relieving to see that my basement walls aren't the only ones that look less-than-appealing =)