With its black color, Black Recluses may be difficult to perceive in the Spider Nest.
Their high damage output combined with their property of being difficult to see and chance to inflict the Venomdebuff for an additional 40 damage, makes them extremely deadly and can easily kill players in early Hardmode.
Because of their low health and high defense, using weapons that deal a lot of damage with a single strike is the best way of defeating them. High knockback weapons can also hold them off for long enough to get them killed.
If a hardmode player enters a spider nest for a short amount of time and exits, there is a 95% chance that up to 10 black recluse can spawn outside the nest, most likely being near the way you entered the nest. To see Black Recluses, you can place Crystal Shards along the way you came in the nest, so the mob's silhouette is visible.
A good tactic on killing Black Recluse is to use a Yelets.
Trivia
The name "Black Recluse" is a combination of the spider names "black widow" and "brown recluse". Both spiders are commonly known for their venomous capabilities. This is reflected in the monster's appearance and its drops.