The best time to double down in Blackjack is when you have a strong chance of improving your hand with one more card and when the dealer shows a weak upcard. This is especially true when you have a total of 10 or 11, as you're likely to hit a 10-value card, or when you have a soft hand (like Ace-5 or Ace-6) against a dealer's weak card. Doubling down increases your bet, so it’s most effective when the odds are in your favor.