These are curtains made from thick and heavy fabrics which are incorporated with special backing. The purpose of these curtains is to block away heat and light from a room. Block out curtains are placed in rooms that may need less light or may need additional insulation. Home theaters are one example of areas where block out curtains may be used. There are a lot of readymade curtains online that one may look from for different types of fabrics or styles.
There are many readymade curtains online if looking for black out curtains and one may also look for secondhand fabrics for these. Block out curtains saves energy because it acts as good insulation during the winter which prevents heat from escaping the room as well as blocking drafts from entering window spaces.
Obviously block out curtains have limited uses and will not be advisable for tropical or humid areas where the temperature can be very hot. Using such type of curtains can absorb heat easily and prevent ventilation to circulate.