The solution to this is can be found in the example "GPIO_EXTI" for the Nucleo-H745 board. That example has "MPU_Config();" in the M7 main.c file. By adding the contents of that function (code posted below) in the same place in my M7 main.c (in main()) the problem was solved. Now the blue button works as expected in the M4 main.c file. BTW I tried adding the function and calling it but I got a few silly errors I didn't want to deal with:
Thanks Tech support!
static void MPU_Config(void)
{
MPU_Region_InitTypeDef MPU_InitStruct;
/* Disable the MPU */
HAL_MPU_Disable();
/* Configure the MPU as Strongly ordered for not defined regions */
MPU_InitStruct.Enable = MPU_REGION_ENABLE;
MPU_InitStruct.BaseAddress = 0x00;
MPU_InitStruct.Size = MPU_REGION_SIZE_4GB;
MPU_InitStruct.AccessPermission = MPU_REGION_NO_ACCESS;
MPU_InitStruct.IsBufferable = MPU_ACCESS_NOT_BUFFERABLE;
MPU_InitStruct.IsCacheable = MPU_ACCESS_NOT_CACHEABLE;
MPU_InitStruct.IsShareable = MPU_ACCESS_SHAREABLE;
MPU_InitStruct.Number = MPU_REGION_NUMBER0;
MPU_InitStruct.TypeExtField = MPU_TEX_LEVEL0;
MPU_InitStruct.SubRegionDisable = 0x87;
MPU_InitStruct.DisableExec = MPU_INSTRUCTION_ACCESS_DISABLE;
HAL_MPU_ConfigRegion(&MPU_InitStruct);
/* Enable the MPU */
HAL_MPU_Enable(MPU_PRIVILEGED_DEFAULT);
}