Tom Avery is a descendant of Damon and Stefan's unnamed half-brother. So yes technically Tom is a Salvatore by blood and he is related to Damon.

But wouldn't Stefan and Damon have known about him, since they kept track of their family members through the years?

^not if someone along the line had a secret child or a secret age affair that led to a child that even that Salvatore didn't know about.

I agree that it would technically make tom Avery a salvatore because he is after all a descendant of Stephan's unnamed half brother. except from the fact that they didn't know about him which doesn't make sense to me but on the other hand of them being related I would mostly agree.