we don't have any idea of age, but blue was a colour used in the pharmaceutical trade, especially - maybe a lead there. And continuing the ramble....pontil marks exist because there was a need to detach the blowing iron, re-attach at the base, and cut/finish the rim in some way or other - the opposite of, presumably, a Bovril bottle that was blown/pressed mechanically, with the entire manufacure performed in 'one hit' (tell me if I'm off the wall with all that, please). It appears that this was blown manually in view of the casing and pontil mark. Agree the rim is unusual, to me it looks almost as though there is a very slightly raised ridge atop the rim, but could be I'm seeing things.
No threading for a screw top - cork possibly?? Perhaps it was a liner after all. How much wear is there on the base Julie?.