Pores themselves can not really "open & close" since they are not a muscle. They are small openings all over skin that house the hair follicles and sebaceous glands covering your body.
Generally pores may seem a little larger when it's hot because when you're hot, you sweat and that sweat comes out of the pores.
You can however make them appear smaller with regular upkeep by keeping skin hydrated and clear of sebum or dead skin cells which can stretch the pore out and make them look larger. If you have blackheads and large pores, you can use the activated charcoal clay masque to clean out your pores and draw out blackheads.
If you physically tighten the muscles around your pores with cold temperatures for example, then the pores in that area can look smaller because the muscles surrounding it are getting constricted from the cold. You can rub an ice cube on your face every morning after using the activated charcoal clay masque, or you can use thepore smasher if the ice is too messy and time consuming for you.
Nope, blackheads are not caused by dirt. If you have larger pores, the dead skin/sebum build-up reacts with oxygen turning the substance black. Thus, the rise of a blackhead!