car tags and bumper stickers that are black with a horizontal blue stripe; i wondered what they are for?

Answer 1

It's a symbol for the Thin Blue Line organization. It is a police organization and charity for families that have lost a family member in the line of duty.
http://www.thinblueline.com/

Answer 2

I have a co-worker who has those on her Jeep. She has them because her baby's father is a deputy. The tags and stickers are supposed to inform other law enforcement, EMTs, etc. that the driver and possibly others in the vehicle are related to someone with a law enforcement career. In the unfortunate case that someone displaying these items is in a wreck, hopefully someone can identify them before their loved one arrives on the scene and help them cope with the situation in whatever way necessary. It apparently also doesn't hurt in the event that you are stopped for a moving violation, improper equipment, etc., at times may even prevent you from being stopped.