I can offer my thoughts: "full-blown" means "fully-developed", so a "full-blown disorder" is a disorder (for example an illness, or a problematic behaviour) that is fully expressed - but it doesn't necessarily mean that it is very harmful (that would depend on the disorder). For example, you might be anxious about some things without having a full-blown anxiety disorder. In this context, a 'full-blown disorder' refers to something that is much more significant, and possibly diagnosed, or meeting all of the criteria for diagnosis.