The common blackhead is a perennial plant with a creeping rhizome. Stems are ascending at the base, 15-30 cm high, hairy in the upper part.
The leaves of the upper pair are sessile or almost sessile, the rest are on petioles. The leaf blades are oblong-ovate, obtuse, whole-edged or indistinctly toothed.
Flowers in false whorls, on short legs. They are collected in apical, dense, spike-shaped inflorescences. The bracts are broadly ovate or almost rounded, often dark purple. The calyx is double—edged, with short sharp teeth, the corolla is also double-edged, purple in color, 1.5-2 times longer than the calyx, with a straight tube, with a hairy ring inside. The upper lip of the corolla is helmet-shaped.