If you want Little Greene's Jack Black in a Dulux tin, go for Dulux Rich Black. It measures ΔE 2.1 from the original — comfortably inside the "very close" band (under 2.5), so the difference is barely perceptible on a wall. Both sit at an LRV of 2.4, which tells you these are proper deep, light-swallowing blacks rather than the softer off-blacks like an Off-Black or Pelt.
Dulux Night Jewels 1 is the other contender, and frankly it's neck-and-neck — also ΔE 2.1 and LRV 2.4. The two Dulux blacks are essentially the same colour by different names, so pick whichever your local stockist can mix without a fuss.
Now, the "but what about" question: should you bother matching at all? Jack Black is a cracking colour and Little Greene's pigment load and depth are genuinely lovely — if you're doing a feature wall or joinery where the finish really matters, I'd buy the real thing. The match makes sense when you're tying into an existing Dulux spec, working to trade pricing, or matching a contractor's standard range. At ΔE 2.1 you won't see a meaningful difference once it's on the wall and dry.
One practical point with any near-black: sheen and undercoat matter more than the last sliver of ΔE. A true black laid over a patchy or warm-tinted primer can look uneven. Prime with a grey or tinted basecoat — most decent paint counters will tint your undercoat darker for deep colours, which saves you a coat and gives you that flat, bottomless black.
And always — *always* — sample both Rich Black and Night Jewels 1 against your actual Jack Black swatch in the room itself, morning and evening. Blacks shift surprisingly under artificial light, and that's where you'll spot whether one leans a touch warmer than the other for your space. Get the sheen level right and either Dulux black will do the job nicely.