Mechanism of redirected blood
A major determiner of blood flow redirection among muscles is increased oxygen demand in the working muscles.

When a muscle works, it uses oxygen, so the partial pressure of oxygen (PO2) in the muscle falls, which signals small arteries (arterioles) to dilate, which results in increased blood flow and hence oxygen and nutrients delivery to the muscle (Skeletal Muscle Circulation, NCBI Bookshelf).



When you exercise your forearm muscles without exercising upper arm muscles, the arterioles in the forearm muscles will dilate but in the upper arm muscles will not. The blood will just flow through the large arteries of the upper arm toward the forearm, but it won't be delivered to the upper arm muscles.

A long-term effect of exercise is the formation of new capillaries in the exercised, but not in other muscles (PubMed, 2008). So, six months of exercising forearm muscles without exercising upper arm muscles will result in better circulation only in the forearm muscles.