I’ll also entertain the theory that since we have no idea how much time passed since the Times Square incident in the happy ending, any number of scenarios could have resulted in Boston having a completely normal pinky finger once more.
Now, just to be fair to happy ending folks, it’s also possible Blindspot simply forgot about the detail of Boston’s finger being missing when filming the finale.
These things happen even with shows with enormous budgets, so it isn’t impossible that the same thing happened here.
Who am I to say there weren’t significant advances in medical technology in that gap?