Bard’s Cutting Words
When a creature that you can see within 60 feet of you makes an attack roll, an ability check, or a damage roll, you can use your reaction to expend one of your uses of Bardic Inspiration, rolling a Bardic Inspiration die and subtracting the number rolled from the creature’s roll.
PHB, emphasis mine
Blood Hunter’s Blood Curse of the Eyeless
When a creature you can see within 30 feet of you makes an attack roll, you can use your reaction to roll one hemocraft die and subtract the number rolled from the creature’s attack roll.
Blood Hunter by Matt Mercer, emphasis mine
Since both features require the use of your reaction, you have to choose one of them. If you can somehow take multiple reactions in a single round, combining magical effects would allow you to use both since they are different effects. As Peter Cordes pointed out in the comments, even Matt’s Cobalt Soul Monk subclass which can have multiple reactions would have to choose one effect for the trigger.