I assume you mean "re-blued" the term for the restoration of an original blue finish on a worn gun. Technically, a gun that was originally plated can't be "re-blued" since it was not blued in the first place.
In order to blue or reblue a gun, any remaining finish has to be removed and the surface polished to remove old patina and surface rust so that the new blue will "take". That means buffing the gun to a smooth surface, either by hand using emery cloth or (normally) using a soft wheel impregnated with polishing compound, called "grit".
When a wheel is used, the grit tends to drag at the edges and corners of the gun, and also in the identifying marks (like the maker's name) stamped into the surface of the steel. That rounds over the sharp edges and "dishes" out the lettering and circular holes. While some viewers might not notice, or might even think the result is pleasing, to the experienced eye those indicators virtually scream "reblue".
Is rebluing bad? Not if the gun is not of collector interest, or was in such bad condition that anything would be an improvement. And of course, a reblue (unless something else was done) will not affect the operation of the gun in any way. But if the gun is one in which a substantial portion of its value depends on collector interest, rebluing definitely reduces the value. One of the prime points in gaining collector interest, and hence a higher price, is the amount of original finish on the gun. Obviously, a reblued gun has zero original finish, though some books arbitrarily assign a reblued gun an original finish figure of 60% or 65%.
Whether an individual buyer considers a reblue job a factor in purchase or in the price paid is up to that buyer. But because most buyers will pay less for a reblued gun, it is often in the interests of the seller to claim that the finish is original or to use such phrases as "beautiful blue finish" or "looks like new." The buyer needs to learn enough about original finishes to be able to make his own determination.
Jim