Agreed, eating wood is typically either a sign of needing more forage or nutritional imbalance/deficiency.
Like LHP said, a mineral block is mostly salt. A typical 4 lb block doesn't contain enough of any of the minerals (except salt, of course) to meet even a day's nutritional needs. If your horse isn't already getting a quality fortified grain (fed at the minimum recommendation on the bag or higher rate), ration balancer, or vitamin/mineral supplement, then it's probably a good idea to add one to her diet. I personally lean more towards the ration balancer (mine gets Triple Crown 30% Supplement) because it doesn't have the extra sugar/starch/calories of a typical grain-based hard feed, and it provides extra protein and amino acids (especially lysine) that you don't get in a vitamin/mineral supplement.