The blackside darter may attain a length of two to four inches. A black spot can be seen at the base of the tail fin. A dark, “teardrop” mark is present under each eye and another dark mark runs from the front of the eye to the snout. The back and upper sides of the body are green-brown with wavy, dark lines and eight or nine dark saddles. The sides have six to nine, large, oval, black blotches. The belly and lower body are white. The first dorsal fin is black at the front and along the base. The dorsal and tail fins have brown lines and black specks. The lateral line is complete. During the breeding season the male becomes very dark, changing color even more when spawning, the iris of the eye turning from brown to gold, and the body and head turning gold or green.
The blackside darter may be found statewide in Illinois. This fish lives in creeks and rivers with a silt-free, gravel or sand bottom. It is not strictly a bottom-dweller as most darters are. It also swims at midwater and the surface. This fish spawns in spring in water about one foot deep where there is a good current over a sand or gravel bottom. The eggs are mixed into the sand or gravel by the spawning actions. No parental care is given. The blackside darter eats immature insects.