I would think (and rule in my game) that she would actually LOOK the same... depending on her Disguise check anyway... just not shedding light. Kind of the reverse of the Invisible light source, she would look like she sheds light, but actually doesn't. More like the picture of the lantern not actually giving off light, it just looks like it.
Well, unless she was an Aasimar or Tiefling or some race that doesn't have the same Type as the "scary fire knight". But if that were the case, then she could disguise self as a fire elemental - which would be totally weird...
SO - IMHO -
"does it look like she has a bunch of continual flame spells cast on her stuff?" Yeah - that's what disguise self does. it makes her look like "a scary fire knight" complete with what looks like flaming armor/clothing/equipment.
"If she's in a dimly lit room, does she shed light?" Nope. No light.
"or does it look like she does?" Yeah. Looks like flames, but no light (just like continual flame doesn't produce heat, smoke, or use oxygen or set things on fire).
As to the Ioun Stone ... I agree with Azothath - " that's a big 'ol can of GM gray area." At first glance I'd say "no - it can't look like an Ioun Stone" - but then I could see BBE making burned out Ioun Stones or Ioun Torches part of their guards uniforms. "I can tell that 'guardsman' is a hero in disguise! No Ioun Stone!"