It is wonderful that you are working with blind senior citizens. Yoga postures are difficult enough with two good eyes. If we close our eyes for just a few moments, we quickly appreciate the challenges facing blind students, as well as their courageousness. Tremendous focus, determination, and attention are necessary qualities of blind students, and these qualities help make them sincere and alert practitioners. It also takes more skill and planning for a teacher to conduct a public class with one or more blind students in it.
As you mentioned, balance is a big concern for blind students. Other considerations include having a sense of reference to the surroundings and layout of the room, and the students' position in that space relative to other students or objects. Just as you might show your house to a visiting guest, take the time to walk these students around the perimeter of the studio, showing and explaining the location of props or other landmarks. This courtesy would help the students feel more at ease and more confident in their new surroundings. Assigning a special place in the room near a wall, and having props already placed there, would help them develop a greater sense of independence and self-confidence. This area should be easy for both the students and the teacher to access.
Your instructions to blind students—as to any students—should be clear, concise, and direct, giving an overview of the pose and a sense of its direction. Tactile instruction, or touching the students with accompanying verbal instruction, may be helpful occasionally. In the beginning, it may help the students' understanding and ability to work independently if you provide some personal instruction on how to do the poses and use wall support or other props.
I hope these ideas are helpful. It requires patience and skill to teach a class with students of various abilities and disabilities. Teaching yoga is a service, and both you and your students will benefit.