It sounds like your chives have been attacked by aphids, tiny pests that love plants with soft, new growth like chives. 

Aphids can be black, green, or other colours and are notorious for multiplying quickly. In the height of the season, female aphids can produce huge numbers of offspring without mating, which is why they seem to appear overnight!

Here’s how to deal with them:

• Blast them away: Use a garden hose with a firm spray to dislodge the aphids. Repeat this 2–3 times a day initially to discourage them from returning.
• Natural sprays: On mature plants, apply a diluted horticultural oil spray. This works well to manage aphids. For an edible plant like chives, soapy water is also effective – especially with repeat applications over a short period of time.
• Keep plants strong: Healthy plants are naturally more resistant to pests. Before planting, prepare your soil by digging in Daltons Compost 1–2 weeks in advance to enrich and aerate it. During the growing season, feed your plants with Daltons Vegetable Fertiliser every 4–6 weeks to keep them thriving.
• Prevention with garlic chives: If you’re looking for a hardier option, try growing garlic chives. They taste like regular chives with a hint of garlic and are more resistant to black aphids.