Quick question, at what stage of an actual herpes blister on mouth stops being infectious? 

Context, 10 days ago I started developing a sore (I have had herpes since a young kid, with sores happening every few years). It has already gone through all stages, including scabbing, which has fallen off several times while taking a bath, etc. As of today, I see whitish "fatty" skin on the borders of where sore was (looks to me like healing skin that has been ripped off), and the interior is now fully flat, albeit reddish, and "mucous". All of this looks like any other healing wound to me now. How infectious is it the site by this stage, compared for example with active sore and asymptomatic stages? Is having oral sex now significantly riskier than when no lesion is present? 

The stage when it is most infectious is when it is in the water blister stage.  I would strongly discourage you giving anyone oral sex until the sore is 100% normal skin.  When we use PCR testing to look at how much virus is in the scabbed stage, for example, we can definitely still find virus present.  And remember that your body can give off virus from the oral area when you do and when you do not have a cold sore.  This is called asymptomatic shedding, when you are giving off virus but do not have a cold sore.  Please let me know what other questions you may have.

Terri