A tablespoon is a non-scientific but very widely used unit of volume, which works very well in cooking, herbal medicine, etc. It is represented by a capacity of one tablespoon which varies from roughly 15 mL in the US to 20 mL in Australia.
1 If tea is properly stored, it can potentially last up to a year for green and black teas. White tea lasts about six months before it isn't as good in quality as when fresher. Stale tea (or bad tea) will taste flat, as if you're drinking paper.  Glass or ceramic containers are fine provided that they're opaque.
Drinking blueberry leaf tea likely wont harm anything but it wont cure you and is unlikely to help you get of medication. Diet and exercise (assuming type 2) are your best weapons to get of medication but even then, not everyone is physically able to. Kdroberts is right, there is no cure for diabetes.