That is BER (Blossom End Rot). See the FAQ linked below and if you need more details the search will pull up lots of discussions.

Primary cause is young plants getting inconsistent soil moisture so the calcium isn't distributed throughout the fruit. As plant matures and IF you stabilize the soil moisture levels it goes away. But if you have more than 1 Brandywine in the same Earthbox IME it will be difficult to do that. Been using EB for several years and they handle 1 big indeterminate well, 2 big indeterminate plants are very problematic.

Meanwhile toss the affected fruit to reduce stress on the plant.

Dave

Here is a link that might be useful: BER FAQ

Thanks! Both plants are about 5 ft tall but one definitely is producing tomatoes better than the other... maybe I will just take the second one out.