Thank you for reaching out to us.
A blister is not considered a barrier to immersion because it is a part of the body. Even wet blood or fluid from the blister is not considered a barrier to immersion. You should, however, be careful to clean the area well during your preparations and to rinse it off again prior to immersion.
We are sorry to hear that the blister is likely to be irritated during your walk. If this is also painful for you, or if you think that it will significantly delay the blister’s healing, please get back to us with more details, in case it would be relevant to try to find a way for you to reach the mikveh by car.
Refuah sheleimah! Please don’t hesitate to follow up with us.