If I have a bard with cure wounds, and take one level of Celestial warlock (Xanathar's Guide to Everything, p. 54) before getting the Boon of Spell Mastery (DMG, 232):
Choose one 1st-level sorcerer, warlock, or wizard spell that you can cast. You can now cast that spell at its lowest level without expending a spell slot.
The Celestial warlock has an Expanded Spell List that includes cure wounds as an option.
Does this inclusion of cure wounds as a possible spell to select, but that I did not actually select, allow for the use of the Boon of Spellcasting with it?
Best Answer
RAW: Yes in general. No for the character you described
The epic boon of Spell Mastery says the following (emphasis mine):
You indicate in your question that Cure Wounds might be a spell that's on your character's spell list due to their one level dip into Celestial Warlock, but your character doesn't know it as a Warlock. If your character doesn't know the spell as a member of the stated classes, then they don't meet the criteria of being able to cast it and thus are ineligible for selecting Cure Wounds for this boon.
That said, were you to select Cure Wounds as a spell known then you meet the criteria of the spell being one you can cast and thus, yes, you can select Cure Wounds for the boon.
The Catch
Once you're in the realm of epic boons, you are also in the realm of DM discretion to prevent game imbalance. As the DMG and the epic boons were published prior to the Celestial Warlock (or the Divine Soul Sorcerer), your DM would be well within their bounds to restrict Cure Wounds from what can be selected for that boon.