If I had to build one all-out melee kit for putting Brutal Black Dragons down fastest in 'Old School RuneScape', I’d focus on sheer single-target DPS plus a way to chew through their defences. My go-to combo is a high-accuracy stab or
crush weapon (depending on your gear) paired with heavy strength bonuses, Piety, and a Dragon Warhammer/Bandos Godsword for the defence drop. For me that usually looks like a 'Ghrazi rapier' for raw stab accuracy and fast consistent hits, or the 'Abyssal bludgeon' if I want heavy crush damage — either of those will outpace most other melee choices on a single target. I slot a 'Dragon warhammer' in the inventory to smash their defence whenever the special is up; that little defence nerf multiplies your DPS over the fight.
Armor-wise I favor a strength-focused setup: 'Bandos' chest and tassets (or the strongest hybrid chest you’ve got), 'Barrows gloves', 'Primordial boots' or 'Dragon boots', and an '
amulet of torture' or 'Strength amulet'. Bring prayer gear (a switch to a prayer-boosting cape or using a 'Fire cape'/'Infernal cape' depending on what you own), and always run 'Piety'. Inventory should be super attack + super strength (or a single super combat potion), plenty of high-healing food like sharks/rocktails, a couple of restore potions for prayer, and an antidragonfire potion or an antifire shield — Brutal Blacks will spit dragonfire.
Playstyle: burst with the Warhammer/Godsword special early to lower Defence, then pound them with rapier or bludgeon while keeping prayers up. If you want absolute fastest, a maxed player with 'Ghrazi rapier' + 'Dragon warhammer' specials timed perfectly will usually net the quickest kills; the bludgeon shines if you prefer higher max hits against their defences. Personally, I love the rhythm of popping that special then watching the HP drop — feels super satisfying every time.