Boric Acid - Mechanism of Acidity
As for acidity of boric acid you can find on https://en.wikipedia.org/wiki/Boric_acid that it dissociates in both ways, but is very weak. However it's acidic enough that it's only a matter of time until it would become almost completely deuterised if dissolved in D2O. It happens because in mixture there would be much more deuterium then normal hydrogen. The acid is too weak to capture or detect its anions. Maybe kinetics of deuterisation would give us some info about properties of its anions.