The trick is going diagonal and keeping it higher than you'd instinctively place it. Skip concentrating on the apples directly - that's actually what can emphasize roundness. Instead, suck in your cheeks and apply blush to that hollowed area, but angle it upward toward your temples in a slight diagonal line. This creates lift and draws the eye outward rather than inward, which naturally counters that fuller appearance.
The placement matters more than technique here. Start the color roughly from your cheekbone (that hollow spot) and blend it back toward your ear, keeping it in the upper two-thirds of your cheek rather than the lower apples. Use a light hand and build it gradually - it's easier to add more than to dial back. A cream blush can be more forgiving than powder for blending seamlessly without harsh edges that might accentuate what you're trying to minimize.
One thing I've noticed with makeup in general is that people often fear being too light-handed, then overcompensate and end up with a heavier application than intended. Start small, step back from the mirror, and assess before adding more. The diagonal lift works because it follows the natural contours of your face upward, and that directional element does way more for definition than the placement itself.