(defn blank? [string]
  (every? whitespace? string))

(defn whitespace? [character]
  (Character/isWhitespace character))

clojure
vs
apache commons java implementation

public static boolean isWhitespace(CharSequence cs) {
5371            if (cs == null) {
5372                return false;
5373            }
5374            int sz = cs.length();
5375            for (int i = 0; i < sz; i++) {
5376                if (Character.isWhitespace(cs.charAt(i)) == false) {
5377                    return false;
5378                }
5379            }
5380            return true;
5381        }