Does /dev/block/mmcblk0 include an external sd card that has been formatted as internal?
No. Internally formatted external sd card is not included in mmcblk0. dd dumps files, in your case the block device file mmcblk0 which includes all partitions, GUID partition table (GPT) and any free space in between partitions.

External SD card is a separate flash memory chip, so exposed by a different block device available at path something like /dev/block/mmcblk1 or /dev/block/sda.