A member of the NPC thieves' guild cannot report murders. If you steal from an innocent player, then there are a few different possible outcomes.
1. Complete Success: One possibility is that you successfully steal an item and go completely unnoticed (in the sense that the game will not officially count any player or NPC as having noticed you even if a player realizes you stole from him or her). If this is the case, you will remain blue. You will, however, be freely attackable by anyone, even in guarded zones. This is known as being "permagray." Once you are permagray, you remain so until you die.
2. Partial Success: Another possibility is that you successfully steal an item but are noticed by a player or NPC. In this case, you will get a criminal flag and turn gray. If you are in town, the guards may be called on you. If the player you stole from attacks you, you will get a murder count for killing this player, even in self-defense. When your criminal flag wears off, you will be permagray.
3. Failure: You do not succeed in your attempt to steal. You may also go gray, as in a partial failure.
IMPORTANT: Do not fight back against a player that highlights blue to you. If you kill such a player, you will get a murder count and be removed from the Thieves' Guild for a week. Only fight those that appear gray to you. If you find yourself attacked by a person that highlights blue, run away. Come back after two minutes and, if the player attacks you again, you will now be able to defend yourself with lethal force. Do note that you will not auto-defend against an attacking player that highlights blue, so you need only use a little care to avoid getting murder counts.
Getting caught and difficulty when Stealing considers the difficulty of the steal (item weight) And the number of humans that are in your view (from server patch notes 77).
From what I have found its working as intended.