Found the answer myself, leaving it here in case someone (like those who called the Platinum line before me) have the same question:

"You can book any available ticket on the Air France and KLM websites using your blue credits, provided that you have enough credits to cover the complete cost, including taxes and surcharges." https://www.bluebiz.com/en/service-c...book-a-ticket/

So yes, it cannot be used for part of the costs.