[RPG] Being blind, flanking and threaten

Let's start with blindness.

All opponents are considered to have total concealment (50% miss chance) against the blinded character.

The scent ability allows a character to pinpoint a target's location, allowing it to make attacks against the appropriate square, but this does not remove the concealment.

The creature detects another creature's presence but not its specific location. Noting the direction of the scent is a move action. If the creature moves within 5 feet (1 square) of the scent's source, the creature can pinpoint the area that the source occupies, even if it cannot be seen.

Finally, concealment means you cannot make attacks of opportunity. (Or, I think, grant/benefit from flanking, but I'm not sure about that one).

You can't execute an attack of opportunity against an opponent with total concealment, even if you know what square or squares the opponent occupies.

From the D&D Glossary: threaten:

To be able to attack in melee without moving from your current space. A creature typically threatens all squares within its natural reach, even when it is not its turn to take an action. For a Medium or Small creature this usually includes all squares adjacent to its space. Larger creatures threaten more squares, while smaller creatures may not threaten any squares except their own.

Source: PHB

As long as the creature could make an attack on its own turn, it threatens a square. As an example of when a character could be armed and not threaten an area, during the surprise round, I would argue that a character does not threaten any squares, because they would not have a turn in the initiative order. Similarly, a petrified or dead creature would not normally be able to attack on its turn, and therefore, would not be a threat.

In your example, since this appears to be a standard round of combat (and not the surprise round), and both Bob and Chris are aware of the threat and engaged in combat with it, Bob would provide a flanking bonus.

Hope this helps.