The plural of Schlumpf is (die) Schlümpfe, and not Schlumpfs. If only plural forms in German were as easy as tacking an “s” on at the end!
While Schlümpfe may have been black in Germany at one point, they are now very, very blue. A toy company still sells plastic Smurf figures in Germany. (My personal favorite is the Smurf in lederhosen.)