You should consider doubling down in Blackjack when you have a strong hand, typically a total of 9, 10, or 11, and when the dealer shows a weak upcard, such as 3 through 6. Doubling down is also an option with soft hands (hands containing an Ace counted as 11), especially when the dealer's upcard is between 4 and 6. The goal is to take advantage of situations where your chances of improving your hand are high, allowing you to maximize your potential winnings. However, with weaker hands or against strong dealer upcards (like 7 or higher), it's usually better to avoid doubling down and instead choose to hit or stand.