You should consider doubling down in blackjack when you have a total of 9, 10, or 11, particularly if the dealer has a weak card (3–6). It's also a good option if you have a soft hand (like Ace + 2–7) and the dealer shows a 4–6. In general, doubling down is most advantageous when the dealer is likely to bust, and you're in a favorable position to improve your hand. Always factor in the specific table rules and any restrictions on doubling down.