My original intent was to give the PCs a chance to get into the room before the darkness acts. It's supposed to be a bit of "The room is utterly dark and no light will penetrate it... what do you do?" before combat begins. When combat DOES begin, you want a moment of "What IS this?" as the darkness becomes solid around you. I wouldn't play it especially tactically sharp (singling out the weakest characters) because it isn't really a sentient thing; it's hungry darkness. The original tactics specifically called out that radiant damage infuriated it and would draw it away from other targets - and again, that it wouldn't venture too close to the edge of the room.
With that said, in terms of freeing an engulfed character who can't get away on his own, with Eberron, my goal is always to think of what would make a good scene in a movie or book, and to reward creative thinking. I'd PERSONALLY allow the artificer to use thunder armor to free himself, because I can see it in my mind... the darkness solidifying around the character and then shattering as the concussive force explodes from his armor. I'd also definitely allow a PC to either help an adjacent engulfed PC escape as a move action (using Athletics against the darkness and leaving the PC adjacent to the darkness) or to grab the PC as a standard action against the defense of the darkness, but then being able to use a move action to run, dragging the grabbed character. Lostsoul's idea is also an interesting one. As it's clearly stated that no light can penetrate the gloom, I wouldn't let a sunrod alone provide complete protection; but I might allow a character flinging a sunrod at the darkness to distract it and give the character trapped inside a bonus to an escape check. Depending on how the scene was playing out, I might allow a character using a radiant attack to spend an action point to make the radiance even more brilliant and add a push effect to that one attack... but I've always been one to allow action points to perform dramatic actions outside the rules.