Looks like normal ageing of the cactus to me, given that there's a lot of fat new growth above a fairly thin section. To counter wind-induced flexing the cactus has grown more woody by the looks of it. Except for the bit on what looks like last year's growth. There it seems the cactus is considering growing more roots because of shading.

Bridgesii seem prone to them black pus-spots. Somehow they're a bit more sensitive than some of the other species.