He was trying to salvage every good coffee beans until all left were these insect bored and black beans.
Based from own experience, these two coffee defects are the most prominent, or the very common defects seen in traded beans. Other defects are common too but these are the more noticeable.
Coffee beans bored, half eaten or almost completely eaten by weevil. If the coffee is to be sold as whole roasted coffee, then including significant number is a major drawback. It is ugly, unpleasant looking and even one in a thousand seen by customer might cause rejection.
More often than not, insect damaged beans gain heat faster, cook faster and burn faster. The desired color is gained while other are half cooked. Burnt while others are just gaining the right roast. The uneven color result.
Weevils leave their discharged on coffee beans, and likely to be included in final product. The coffee coming out from alamid anus are collected and processed to a very expensive product. Do not expect the same for coffee with weevil poo.
Improper storage causes weevil attack. Maybe the right way to store coffee is packing a weevil-free and sufficiently dried coffee in air tight and durable packaging. A nice but expensive method.
The black beans. According to zeccupa.com, black beans are caused by: 1) Insect or bug attack which result to slightly discolored or black beans, the term antesia. 2) Harvesting immature beans and dead cherries. and 3) Long exposure to heat or water.
Black beans depending on cause affect coffee flavor differently. Insects contribute off flavor. Immature beans have undeveloped flavors. Length of heat exposure might be enough to partially cook the beans, resulting to burnt beans during roasting. Water soaked coffee are denser and harder to roast. Moisture content if too high allows mold growth.