that all depends. topographically speaking, elk and blue mtn are different. elk is more like a true peak with ridges radiating out from the top. blue is a ridge line with no real peak. so basically, it really depends on what kind of mtn you like to ski on. the one-to-one comparison of elk to blue for terrain isn't the best.
that being said, the snow quality of elk mtn is vastly superior to blue mtn. also, the trail layout at elk has fewer switchbacks (i personally don't like a lot of switchbacks). so i think this is a valid one-to-one comparison...and elk mtn wins hands down.
i have not yet been to blue knob. from what i've seen online and thru trailmaps/ google earth, the layout kinda reminds me of jack frost but the actual terrain is more like montage (flat up top, steep at the bottom). granted, blue knob does have the highest summit elevation of any ski area in PA....so that affords them the ability to blow fairly descent snow. not sure how much natural snow they get compared to the pocono's or elk mtn. one of these days i'll get to blue knob....but if i'm gonna drive 3.5 hrs to go skiing i'd rather hit up hunter or windham in the catskills.