Just put a vinyl blackout roller up and keep the curtains. It will work well and when the roller shade is up, it’s barely noticeable. Guests can choose to use the curtains, the roller shade or both.
An internally mounted blackout roller shade will still have considerable light leak, but the curtains will help and it will be plenty dark enough for guests imo.
If you have a small child visiting you might need to get more aggressive but you can always add a $10 paper blackout shade to the outside of the window in that event.