When tissues that are normally red turn blue, this is because there is less oxygen in these tissues. This can have several reasons. For example when the tip of the tongue of a pigeon is cold, there will be less blood circulation and the tip will appear blue. It doesn't have to mean anything. If it's associated with blue flesh it can be a problem certainly for birds that have to perform. Again it can be caused by multiple factors like inadequate nutrition/supplementing, inadequate training, dehydration (shock in extreme cases), etc...