Description from Flora of China
Sinapis nigra Linnaeus, Sp. Pl. 2: 668. 1753; Sisymbrium nigrum (Linnaeus) Prantl.
Herbs annual, 0.3-2(-3.1) m tall, sparsely hirsute at least basally. Stems erect, branched above. Basal and lowermost cauline leaves with petioles to 10 cm; leaf blade ovate, oblong, or lanceolate in outline, 6-30 × 1-10 cm, lyrate-pinnatifid or pinnatisect; terminal lobe ovate, dentate; lateral lobes 1-3 on each side of midvein, much smaller than terminal lobe, dentate. Upper cauline leaves petiolate, lanceolate or linear-oblong, to 5 × 1.5 cm, base cuneate, margin entire or rarely dentate. Fruiting pedicels straight, slender, erect or ascending, subappressed to rachis, (2-)3-5(-6) mm. Sepals oblong, 4-6(-7) cm × 1-1.5 mm, spreading or ascending. Petals yellow, (5-)7.5-11(-13) × (2.5-)3-4.5(-5.5) mm, ovate, apex rounded; claw 3-6 mm. Filaments 3.5-5 mm; anthers oblong, 1-1.5 mm. Fruit linear or narrowly oblong-elliptic, (0.5-)1-2.5(-2.7) cm × (1.5-)2-3(-4) mm, 4-angled, sessile, subappressed to rachis; valvular segment (0.4-)0.8-2(-2.5) cm, 2-5(-8)-seeded per locule; valves with a prominent midvein, slightly torulose; terminal segment stylelike, sometimes narrowly conical, (1-)2-5(-6) mm, seedless. Seeds dark brown, gray, or blackish, globose, 1.2-2 mm in diam., minutely reticulate. Fl. and fr. Apr-Jul. 2n = 16*.
The above first record from Xizang is based on Thomson s.n., 14 Aug 1847 (K).
On the basis of recent molecular studies and critical reevaluation of morphology, it appears that Brassica nigra should be retained in Sinapis, as was originally described by Linnaeus.
Brassica nigra is a cosmopolitan weed. It is cultivated in the West primarily for the use of seeds in seasoning and pickling, but it was used extensively in the manufacturing of table mustard before it was replaced by B. juncea.
Slopes, steppe, field margins; 900-2800 m. Gansu, Jiangsu, Qinghai, Xinjiang, Xizang [Afghanistan, India, Kashmir, Kazakhstan, Nepal, Pakistan, Russia, Sikkim, Vietnam; N Africa, SW Asia, Europe; cultivated and/or naturalized elsewhere].