Blending is the most basic means of avoiding the attention of guards in the Assassin's Creed Series. Though the specifics have changed between games, the fundamental principle remains the same: acting casual will draw less attention from others.
Assassin's Creed I
Holding A while in Low Profile (not holding Rt) will cause Altair to imitate a monk, causing guards to mostly ignore you. However, you will walk slower while doing this, which could lead to more interactions with guards, reducing the effectiveness of this technique to often counter-productive levels. Use blending sparingly, when necessary.
Alternate Blending (see Hiding Spots)
Blending on Benches
Walking into an occupied bench while in Low Profile will cause you to sit on it, making you less noticeable to guards. However, if the previous occupants become scared due to violent activity, they may stand up or run away, leaving you exposed. (ex- if you provoke a group of guards, then flee to a bench, citizens may evacuate the area, forcing you out of hiding)
Blending with Scholars
Walking into a group of scholars, then pressing A, will cause you to enter their midst and appear as one of them. This will cause most guards to ignore you, and can even grant you access into Restricted Areas. (see Breeching Methods) The small list of drawbacks includes the dependency on the path of the scholars, forcing you to follow them or abandon blending with them. The other limitation is the requirement to locate and contact a group of scholars.
Ending a Blend
When executing a Standard Blend, (holding A in Low Profile) the blend will continue until A is released. When the blend is terminated, all immunity it grants you reverts to normal. Additionally, some extra attention will be drawn, likely as a reaction to the realization that you may not be a monk. Attempt to do this under low-pressure circumstances, such as away from guards.
-Less attention drawn to you
-Can be used to avoid conflict, even after guards have been provoked (to a small degree)
-Slower movement speed
-Terminating a blend will draw additional attention to you