Cuchillo - I’m not making it up !! “blue stain…. will not blow the varnish “ – I think it will -

Here from my BRE ‘rot’ book – there are photos similar to AlistairM’s but I’m no good at loading them:

“Blue stain in service fungus

Commonly - Aureobasidium pullulans and others

Damage characteristics -

Disfigurement of the wood and especially of clear finishes; early failure of surface coatings caused by rupture of the fruit bodies.
Discolouration caused by sap stain which has occurred in the log may still be detectable after drying and conversion of the timber…… Damage to coatings will occur through the growth of blue stain in service.

Cannot grow below 20% moisture content

Treatment - …. Dry the timber , treat with preservative containing an additive specifically active against these fungi….: apply new coating….’