Is it technically possible for a tiefling to use its darkness to affect a large object such as himself? If yes, could this be treated as a stealth bonus in the dark? My level 7 tiefling Dex-based stealth fighter would find this really useful.
Most GMs will probably rule that the tiefling can't target its own body with its darkness
According to rules, the typical tiefling's spell-like ability darkness works just like the 2nd-level Sor/Wiz spell darkness, and that spell has as its target object touched. As the typical tiefling isn't usually an object but a creature, it would take a liberal GM to allow the tiefling to slap the darkness effect directly on itself rather than than, for example, a weapon it's wielding or its armor. However, once the darkness effect's in place, it may be possible for the tiefling to hide within the area.
You can learn more about light levels (which are fairly complicated and resist a simple summary) here. The area's light level must be at least dim (i.e. granting a 20% miss chance due to concealment) for the typical creature to make Stealth skill checks to hide within. The tiefling spell-like ability darkness reduces the light level in the affected area by one step.