It's been ages since first reading this Romance, but, let's read the entire verse:
Himself he wolde have doo to deth,
And to hert he had it smeten,
Ne had his moder it underyeten.
Then the queene fel him uppon,
And the knyf fro him noom.
Floris (still a child, of course) is so distraught at finding, (upon his return) that his beloved Blanchelfour is apparently dead. The scene occurs at her supposed graveside. He wishes to kill himself but has only a childish knyf (Next verse: She reft him of his lytel knyf,
And savyd there the childes lyf.) which is taken from him by his mother.
The doo in question should be transliterated at "He would have struck(himself) to death"...
This is supported by reference to Erik Kooper in The Consortium for the Teaching of the Middle Ages, University of Rochester...
Occam's Razor applies to the "was" you also reference... simply put it means 'she derived from good ancestors'...