A GL Ring can actually manipulate the entire electromagnetic spectrum (which includes radio waves, microwaves, infrared/thermal radiation, visible light, ultraviolet light, X-rays, X-rays, and Gamma rays, and most other forms of radiaton, including Kryptonite-Radiation).
And if you want an Energy Blast to have 2 seperate energy effects, but one Energy Blast with another type as an Extra. Example: Darkbolt (Energy Blast/Darkness 5 w/ Energy Blast/Cold 5 as an Extra. Cost 3pp, total cost 15 points). Whoever's hit with it makes 2 Damage saves, both at DC 20 -- once for the Darkness part, once for the Cold part.
As to how Darkness can hurt someone -- for those of you who play D&D, think of it as Negative Energy damage. For those of you who're avid Marvel comics fans, look at the characters Cloak and Darkstar. For those of you with pseudoscientific minds, think of it as a "Blacklight Laser" (yes, I know Blacklights are really UV lights, but they had Cold Lasers in Godzilla vs. Destroyah, dangit!!!).
FNORD is Fnord is fnord...