It is common enough for those packs of estimation cards to include one with a question mark on it, to denote the "I can't estimate this sight unseen" scenario. The reason it's common is because often people are presented with stuff that isn't well-understood enough, and it's a very good idea not to pull a number out of one's rear in those situations. Don't be afraid to use that card (or do the equivalent and just say so).
If you are afraid because you will be marked as 'not a team player' or whatever, you have management culture problems that neither Scrum nor any other imaginable software development process will fix on its own. They will all end up feeding an idiotic garbage in/garbage out cycle of overpromising, crunch, burnout and general misery, with the superficial appearance of whatever process it was supposed to be.