I have a blocked nose and I can’t feel the breath sensations. Should I put my finger to feel the breath? Is it ok if I focus on other things like finger touching each other? How do I feel the breath sensations if my nose is blocked?
The purpose of formal meditation is to limit our experience to the fewest number of objects in order to allow for easy observation without becoming overwhelmed or distracted. When sitting still, the whole body is tranquil and the only movement is when the breath enters and leaves the body. When the breath enters the body, there should be a rising motion in the abdomen. When the breath leaves the body, there should likewise be a falling motion. If the movement is not readily apparent, you can put your hand on your abdomen until it becomes clear.
If it is difficult to perceive the motion of the abdomen even with your hand, you can try lying down on your back until you are able to perceive it. Difficulty in finding the rising and falling motion of the abdomen when sitting is generally due to mental tension and stress; if one is patient and persistent in the practice, one’s mind and body will begin to relax until one is able to breathe as naturally sitting up as when lying down.
The most important thing to remember is that we are trying to observe the breath in its natural state, rather than forcing or controlling it in any way. In the beginning, the breath may be shallow or uncomfortable, but once the mind begins to let go and stops trying to control the breath, the rise and fall of the abdomen will become more clear and allow for comfortable observation.
It is this rising and falling motion that we will use as our first object of meditation. Once we are able to observe the motion of the abdomen without difficulty, it will serve as a default object of meditation for us to return to at any time.
Maybe try to troubleshoot the causes to the blocked nose first. If it's just normal nose booger, gently blow your nose before the sitting to clear nasal passage. If it's allergy/inflammation, eat high vitamins fruits/veggies and maintain regular daily exercises. Anyway, usually once you've gotten into some relatively settled absorption state, the allergy/inflammation tend to naturally subside and you'll breathe better.
Most Buddhists interpret MN 135 to mean blocked nose is due to bad evil kamma performed in past lives, as follows:
There is the case where a woman or man is one who harms beings with his/her fists, with clods, with sticks, or with knives. Through having adopted & carried out such actions, on the break-up of the body, after death, he/she reappears in the plane of deprivation... If instead he/she comes to the human state, then he/she is sickly wherever reborn. This is the way leading to sickliness: to be one who harms beings with one's fists, with clods, with sticks, or with knives.
According to the viewpoint of these Buddhists, it seems unknowable how many lifetimes, possibly billions, it will take to resolve such past kamma and attain jhana. It seems attaining jhana is very difficult, even impossible, with blocked nose. It would commence with cultivating metta towards all living creatures, large & small. Also, vegetarianism might help.