Just my experience. I am not a licensed electrician.
You got false positive result from your electricity tester. Trust your meter.
When there are 2 black wires (hot wires) close to each other, if one is hot, and another is dead, the dead one still get "induced" by the hot wire and show positive by tester----false positive. If you use meter to test, sometimes it shows 20-30 volts too, but not 110 volts. So you know it is dead wire but get "induced" by another hot wire nearby. It is much safer to trust your meter.