why would someone put a black tar like substance in our front door keyhole?
Answers:
This could be a plain old case of vandalism. Sorry about your luck. I would take the lock to a locksmith (cheaper than having them come to you) and have them rekey it. If other locksets work using the same key, I would have them changed too. The locksmith can tell you if someone tried to make an impression or if it was vandalism.
Good luck with your problem.
Other answers:
They might have been making an imprint for a key.
They might have been making an imprint for a key.
Get your lock changed. I sounds like someone is trying to make a duplicate key.