Loomis,
To check your boat to see if you need to consider a black box or not is easy to do and you can do it yourself. All you need is a digital voltmeter that will read DC. Get out on the water and drop your downrigger ball about 10 feet. Ensure everything in the boat is shut off including the motor and then measure your voltage with the negative voltmeter lead on the negative terminal of the battery and the positive on the downrigger cable. You should read between .6 and .8 volts dc. Then start your motor and turn on all your electronics that you would normally be running and your voltage shouldn't change more than a half a volt or so. If it does you need to check your grounding on your electronics that you have installed. And if all of the those readings that you make are within .6 to .7 volts DC, in my opinion (emphasize opinion) you don't need a black box.

I will also add that I was at the Sportsmans show in Puyallup and went to the Downrigger Fishing Seminar held by Tom Nelson from Salmon U. I told him that my results from the above test had me at .67 Volts DC with all my electronics on, and he also said that I probably didn't need a black box.

One more thing to keep in mind is that those voltage readings will vary slightly from day to day depending on certain conditions around you such as other boats around you or your zincs aging and many other factors that can slighty vary that voltage.

Hope this helps.