That is not annealing on the front of your slide. The front end of the slide was tempered to harden it against wear and battering.
The heels of some of the early WWII M1 Rifles were annealed to soften them to prevent cracking from firing rifle grenades. Both the tempering and annealing darkened the affected metal. Blueing does not make the tempering show up as much as a phosphate finish does.