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Discoloration of a compressor wheel can come from crank case ventilation. So the color of the wheel is no sign of its integrity.
Another thing you have to consider with erosion is that it may not happen on all blades evenly. In fact if you take a close look at that picture you can see that some of the blades have a much more rounded corner than the others. Although this difference may not seem like much this will throw the rotating part of the turbo out of balance and lead to excessive wear on the bearings. As turbos spin up to 200K rpm this has to be considered.
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