The most important products to consider are blinds and curtains that are blackout for maximum protection from light. You are giving yourself additional coverage by doing this. There are many benefits to having blackout. It acts as a shield against light whether it be sun or artificial, it has thermal properties meaning in winter it helps to keep the room warm and in summer keeps the room cool. If your curtains are lined in blackout then it assists in protecting your fabric from fading and perishing (if you have a natural silk).
Blackout roller blinds
The best style of blind to consider is a blackout roller blind fitting inside the recess as far back to the window as possible. Vertical and venetian blinds can be installed but if you want maximum light blockage then a roller is the best product as it is a singular length of cloth.
The blind will reduce the light flow but be aware, you are always going to get some light coming around the edges of the blind. Vertical and venetian blinds consist of vanes/slats that overlap which can cause light to penetrate in between each, so note, you may not get complete light blockage when the blind is in the closed position. Be sure that the blind clears any handles that protrude out into the window.
To reduce the light further then introduce curtains to the window. A track will fit more flush against the window. Aim to fit the track high above the recess enabling less light to filter through and take it slightly further past the recess width than perhaps you usually would. If you prefer curtain poles then do be aware that the brackets can protrude out from the wall more than a track bracket. Consider your heading choice.
Pencil pleat are less likely to let the light through along the top on a track but could cause more on pole as they hang slightly below on the rings. Eyelet heading on a pole will be a better option as you will always have a step up of fabric along the actual eyelets themselves. The length of the curtains can make a difference - choose floor length to guarantee less light as oppose to slightly below the sill.