FYI, i been in chardok around ent up till bridge when i was 48, and most of the naks were blue, so i'd say 45-52, cause there were red and yellow ones among them. last night i dinged there right before a raid, so some of the mobs deeper in may still be 48-52 (naks), no way of telling at 49 :)
I got summoned by one of these, which was particularly unpleasant, as I was trying to solo in Chardok as a druid. I'm just glad I had a natural distrust of reported levels and pulled to zone before rooting and dotting.
Typically see these guys lil closer to named mobs like korokust etc but can have them close to zone. It's just more rare. And yes these boys can be lvl 51+ and can summon about half the time.
I'd have to guess 48 to 52 if I had to put money on it.