Welcome Thistlebunny.
All birds caught in the nets used for monitoring species are ringed. Many of our winter blackbirds are migrant visitors from Scandinavia and are not the same birds that we see in our gardens in the summer. Ringing programmes are one of the main reasons we know this and such monitoring will also give a guide as to how the species is doing in different areas. However, I've yet to see a ringed garden bird. A lot of species that were common in my childhood and taken for granted are much rarer now. I'm lucky enough to have several thrushes but haven't a single sparrow in my garden and these were very common in the 70s and 80s.
And unless you got close enough to see the code I doubt you'll be able to find out where it might have been ringed. Could be worth finding out if any of your local reserves have ringing programmes though.
Hope you enjoy the forum