blue light arround glass tube its risk?

And FWIW the Tung Sol 7581 tube ( 6L6 / KT66) has the prettiest blue glow I’ve ever seen. Sounded good too.

I’ve always liked the blue glow. I think my Quicksilver Monos did it. I’m almost certain my Fender Vibrolux tubes did it. What were those tubes anyway? 8417’s?

One of my RCA Globe 45 tube (from the 1930s) dead soon after the purple glow show. 

I had a bright blue glow in one of my KT 120 tubes on a Cronus Magnum Integrated.  Then it turned bright orange and poof. Tube was gone.  I Had to replace the fuse for that tube and the tube of course.  I would definitely keep my eye on it. 

The blue glow can be natural if the plate voltage is very high, usually above 500VDC+  - for example, all my 807 power tubes glow blue with 700 VDC supply, and I am getting an average of 10,000hrs lifespan out of them. In all those cases when it's natural, it happens from the get go, the tubes have the blue glow right away or develop it during their first few hours of life. Sometimes it can fade a little over time, but it does not get more prominent over time.

However, if you have been listening to a tube for 100s of hours and then it turns blue out of nowhere, that is likely a danger sign.