Many thanks Marco!

This did the trick finally.
I have started this "migration" project from Paradox to Oracle actually in 2018 with Delphi 6 and by then these BLOB fields were added to the TOracleDataset as TMemoField.
Removed the Memo fields and added them again to the TOracleDataset fieldlist. Voila: now in Delphi 10.3.3 these are of type TWideMemoField.
And all works as expected now.