Wet rot ? I think you are right.

On closer inspection there would seem to be more than one type of rot on the go.
The problem with blue stain is that it lies dormant as spores which then errupt as small flecks as in the photos, these break open the protective coating and let in more water and so white, brown, wet & dry rots ( take your pick !) are able to get a hold.
So the fact that it, in itself is not structurally damaging, doesn't put it on the 'harmless' list .....and it loves hardwood as well - spalted beech and ash being classic examples.