The form class takes care of which item is selected by the stored values in configdata field. If you follow the class inheritance through you'll find some function that sets the data. The bit you're looking at is just the form definition.
In the block_instances table was there nothing in the configdata field? You should see a base64 string (a jumble of letters and numbers). Paste the contents into https://www.base64decode.org/ and you should see the serialised array.