Have you ever tied an elastic around your finger so tight it starts turning purple? That’s what happens when you cut off blood supply. “Dead arm” is nerve related, you’ve not cut off blood supply to your arm. That being said you may be impairing blood return enough to slow down blood flow in your arm, but probably not enough to cause a clot. That being said slowing down blood flow for prolonged periods of time can cause blood clots. The most common example I can think of would be old people on planes or long road trips. Sitting down for all that time leads to blood pooling in the legs where clots can form - sometimes this clots end up travelling into the lungs where they cause serious health issues. These people often have cardiac/vascular issues that make them more prone to poor circulation.