Black dragons practically exist to be fought—they are evil to the core, have acid breath, torture prisoners, lord over boggards, kobolds, and lizardfolk, and try to enact genocide on almost everything else. So the tricky part isn’t getting players to want to kill black dragons; it’s simply getting PCs into the swamps in a way that isn’t cliché.
Most black dragons kill any warm-blooded humanoid in their swamps. Zheros the Jet Talon dimly recognizes that the presence of the coastal trading town of L’Onzal brings fat, wealthy prey closer to his swamp. So he tolerates the town…but once a month he paws through the main streets, corrupting the water wherever it strikes his fancy. Should anyone open an alchemist’s or an apothecary’s shop, he is sure unleash his magic there first.
Sadism is a black dragon’s calling card. When a cult of the goddess of torturers opens shop near Blackfens, they contact a young black dragon to aid them in their rites—driving their devotions to new ecstatic heights in the eyes of their goddess while filling the dragon’s pickling pools.
Some black dragons do have a single soft spot: bards. If a bard is abducted near a swamp, chances are a black is involved—and sometimes, they’re even released unharmed. The one-horned Oxipitus is particularly prone to abducting female elven bards because they live so much longer…if he is careful with them, which these days (thanks to his penchant for face-maiming) he is not.
—Pathfinder Bestiary 92–93
Mike McArtor’s Dragons Revisited indicates that blue dragons are the most likely to have half-dragon offspring. But a quick glance through published adventures easily hands the title to blacks, who seem to have bred with anything vaguely reptilian. (In 3.5 their rutting even spawned a new race: Monster Manual III’s blackscale lizardfolk.) A simply way to reconcile these facts: Blue dragons are much more likely to breed strategically, to gain loyal lieutenants, servants who can move around human societies, and as en expression of dominance. Blacks breed out of sheer lust divided by a narrow range of opportunities. Hence blues being more likely to carefully breed across many species, while blacks stick to the reptiles in their swamps (crocodiles, basilisks, dinosaurs, lizardfolk, etc.) who sate their desires and aren’t likely to raise offspring that could eventually become a threat.
By the way, the look of black dragons has changed a lot over the years, from Dragonlance’s sleek Clyde Caldwell-painted black to Todd Lockwood’s 3.0 “skull dragons.” In the Bestiary, Ben Wootten goes for a practically antler-horned, fan-fringed beast that’s pretty cool and definitely evil.