Fruit – How far in advance can I prepare a fruit cup

Probably the easiest way to do it is to just gently pour the blackberries into a basket strainer, then lower the strainer into a bowl of water. Then you can just lift the strainer out and the berries will come with it, no fishing required.

I've preserved cut fruit for at least 6 hours before using just lemon juice. Lemon juice inhibits the oxidation of the fruit which prevents browning as well of a loss of crispness.

One lemon should be enough juice for a 1.5 quart bowl of cut fruit. Simply squeeze it over the fruit and toss gently to prevent bruising.

Since you'll be working with apples, kiwis, and bananas I think your best bet will be to keep them separate. The lemon juice will benefit both the apples and bananas, but might only be marginally effective if unnecessary for kiwi -- since kiwi brings plenty of its own citric acid to the table.

The fruit should be stored in the refrigerator to additionally inhibit oxidation. In my experience covering it with plastic wrap with a few holes poked in it works great. Aluminum foil should be fine as well. I find that the covering is primarily to protect your fruit from picking up other odors from within your fridge, and the holes permit the fruit to "breathe" releasing the buildup of ethylene gas that will occur if it is completely sealed.

If lemon juice is undesired you can use other citrus juices including: lime juice, orange juice, and pineapple juice. Obviously they will all bring their own flavors. If this flavoring is altogether undesired, then you should consider getting a commercial product called Fruit-Fresh. It's basically a powdered citric-acid, and kosher to boot.