Hall-Effect sensors require a ferromagnetic target - your blades are aluminum, and there would appear to not be a way to attach a magnet, or steel object to them to take advantage of eddy currents as the aluminum blades pass by. Assuming you are wanting to measure at full RPM and not on the bench.
However you might be able to use an inductive sensor, as described here?
http://www.compad.com.au/cms/autosys/articles/129 http://www.ia.omron.com/product/fami...index_fea.html
Even if blade distance(s) were known, it would not necessarily be helpful when balancing the rotor?
Rather than measuring the distance you might only need to detect the closest approach of any blade to the outer casing, or whether any blade approached closer than a given clearance?
If the inductive approach won't work, it seems like ultrasound or optical would be the way to go. I hope these links are helpful:
http://www.iop.org/EJ/article/1742-6...7-9ae896a46c3b
http://www.laserfocusworld.com/articles/257213
http://gltrs.grc.nasa.gov/reports/19...999-209183.pdf