Lavender does turn black when it is left in the rain and it rains for days. It does need to be pruned annually but only prune new wood. Cutting into old wood sometimes causes die back. I have killed a few lavender plants myself by over pruning. I cheat now and only prune half the plant and wait to cut back woody stems until they have sprouted new branches. They will only do that well once they are are exposed to more light. Then I cut back the other half. I have had my plant die back before, sometimes one branch, sometimes all of them and usually it is because of roots rotting from too much water.
Happy gardening in Hawaii. Gardens are where people grow.