The choice is huge. Porthole bungs are designed for plugging round portholes. Specialist companies offer customised blind manufacture and a myriad of suppliers will make curtains.
Bungs - these are great for total black out in portholes. Some manufactuarers also produce these to go in to door windows.
Curtains - the choice of fabric and colour is a major benefit for choosing curtains on your canal boat. Often these can be purchased ready to hang in your windows. Due to the sloping sides of a canal boat you will need to consider how they are secured, often a pole/rail is used along the bottom of the window to secure the curtain in place.
Blinds - here you have the choice of Roman, Venetian or roller blind. Unless you have large windows on your canal boat, Roman blinds are unlikely to be the ideal solution as you lose too much light when the blinds are raised due to the pleating. Roller blinds, similar to curtains can easily be secured using a rail/pole at the base of the window. Venetian blinds are ideal in providing privacy without blocking all light out and can be angled as such that you can see out but walkers on the towpath can't see in! There are various methods available to secure the venetian blind to follow the contours of the boat.
Shutters - these are most commonly seen over portholes, but can be used on standard windows too. Similiar to a window bung, the shutters provide a black-out option.