You don't need the exact amount, you can make more books. The term of taking a blind in spades simply means you are taking the gamble that you would bid those many books before actual seeing the cards. Some people play going blind 6 or taking a blind 7. Regardless of which version, you play it simply means you are going to make 'at least' that number of books. In other word, you can make 8 books after taking a blind and you still get the points from that blind plus two over. The only time you can be set back in Spades for making more than your blind is if you double your books. Though this condition is true for any bid.