I like the idea of growing blackberries in addition to the apples. It would give you an earlier income and also help build your customer base. The people picking the blackberries will also be the people picking the apples later.
I would check to see if there are any apple growers in the area. You would at least want to make sure the apples grow and produce in your area. We had a peach orchard go in our area about 15 years ago. The problem is peaches are very hard to grow here. It went under pretty fast. However, blueberries grow well here and we have several people with u pick farms. One person even put in a pear, grape, blueberry and Kumquat U pick place that seems to done well.