If a blue litmus paper remains blue after dipping into a solution, then:
Solution is either basic or neutral.
Solution is either basic or neutral.
As base changes the color of red litmus to blue, so if a blue litmus is dipped into the basic solution, the color does not change.
Similarly if the solution is neither acidic nor basic i.e.neutral, then blue litmus would remain blue as neutral solution does not change the color of litmus.