Hello and welcome to the forum.
It’s advised to avoid psyllium and especially the husk because of its sharpness and hardness. It can actually cut into the intestinal wall causing bleeding. Softer fibers would be a much better choice, such as oat or rice bran.
Blackouts and dizziness can happen during a fast because of reduced blood flow and a lowered blood pressure. Sometimes they can occur on just a one-day fast, and even meal skipping for some people. They usually stop or reduce in severity if you increase your blood sugar. That’s why fainting and dizzy spells are rare during a juice fast unless there are longer periods between taking juice.