What could possibly make finding out that you're a necromancer any worse? Discovering that your body is coated entirely in a creepy, blue aura—which is exactly what happens to Sam when he meets Douglas at the Woodland Park Zoo for their initial meeting.
When Douglas enables Sam to see the auras of living things, he notices that Douglas, the zombie panda, and even himself are all coated in an "icy blue […] broken up with shifting, swirling lines of blacks, grays, silvers" (8.71). In other words, if Sam could bust out in song to share his emotions, he'd probably tell us, "I'm blue."
So what's up with the blueness, huh? Easy. Blue represents the connection a necromancer has to the world of the dead. "You're not like everyone else, Sam," Douglas explains. "Necromancers are linked to death […] You are one of the ties that binds this world to that" (8.78). And while nobody else can see it, the blueness makes this super clear to Sam.
But that's not the end of the story. The quality of the blue aura can also represent the power the necromancer holds. Take the scene where Sam raises Ed from the dead, for example. Sam describes that while his circle is "a dimmer electric [blue]" and Douglas's is "a vibrant ice color," Sam's shade of blue is ultimately "richer." At this point, Sam's powers are still bound, limiting the scope of what he's able to do—but the richness of his aura seems to demonstrate that his power, even in its suppressed state, is greater than Douglas's.
And, of course, his ability to raise Ed proves this to be true.