Diagnosis: Melanochromis robustus has a basic melanin pattern consisting of two longitudinal black stripes on the body, of which the mid-lateral stripe reverses colour with the background in the breeding male, a character that makes it a member of Melanochromis (Ref. 89864). Female M. robustus are distinguished from all other congeners, except those of M. melanopterus, M. vermivorus, and M. mpoto, by a gray to brown coloured body, which is white to yellow in other congeners; mouthbrooding females may have additional vertical bars on the flanks, but these are more diffuse than the horizontal stripes (Ref. 89864). Both male and female M. robustus are distinguished from M. melanopterus, M. vermivorus, and M. mpoto by a broader interorbital breadth, the width of which is 26.1-29.6% of head length, vs. 16.6-20.6% in M. melanopterus, 18.5-25.5% in M. vermivorus, and 14.8-21.8% in M. mpoto (Ref. 89864). Male M. robustus in breeding colouration are distinguished from all other congeners by a dark-blue body colour and two light blue horizontal stripes on the flank superimposed with dark-blue to black vertical bars; the bars are interrupted by the light-coloured horizontal lines; none of the other known species of Melanochromis has a male colouration where the horizontal and vertical elements of the pattern are expressed simultaneously; superficially, breeding males of the sympatric and smaller M. heterochromis have a similar colouration, albeit lacking the vertical bars; Melanochromis heterochromis can also be distinguished by its light-coloured mid-lateral stripe, which rarely spans more than two scales but is usually three to four scales deep in M. robustus; Melanochromis robustus is further distinguished from M. heterochromis by a longer snout, 33.8-38.0% of head length vs. 26.8-31.5% in M. heterochromis, and by a deeper caudal peduncle, 12.9-14.4% of standard length vs. 9.8-11.7% in M. heterochromis (Ref. 89864).

Description: Medium-sized to large mbuna, ovoid body with greatest depth at about second to fourth dorsal spine; dorsal body profile with gradual curve downward, more acute towards the caudal peduncle; ventral body profile slightly convex between pelvic fins and base of rays of anal fin with upward taper to caudal peduncle; dorsal head profile rounded, with continuous curve between interorbital and dorsal-fin origin; horizontal eye-diameter greater than preorbital depth; majority of eye, along horizontal axis, positioned in anterior half of head; snout straight to slightly concave in some individuals; jaws isognathous; tooth bands with 3-5 rows in lower and upper jaws; teeth in anterior outer row unequally bicuspid with lateral teeth primarily unicuspid, and teeth in inner rows unicuspid; at junction of outer and innermost row teeth as single series of small unicuspids (Ref. 89864). Dorsal fin with XVI-XVIII spines and 8-10 soft rays; anal fin with III spines and 6-8 soft rays; first 3 or 4 dorsal spines becoming gradually longer posteriorly with fourth spine about 1.5 times in length of first; last 13 spines becoming slightly longer posteriorly with last spine longest, about 3 times length of first; soft dorsal with in females rounded tip or in males rounded to subacuminate tip, third or fourth ray longest, reaching approximately base of caudal fin in females and approximately 1/4 length of caudal in males; anal spines progressively increasing in length posteriorly; third or fourth ray longest, reaching base of caudal fin; 1-3 small yellow spots on posterior anal fin in females and 2-10 yellow spots on posterior anal fin in males; caudal fin subtruncate to emarginate; pelvic fin not reaching anal fin in females, but extending to second or third spine of anal fin in large males; pectoral fin short and paddle-shaped, reaching superimposed vertical line through base of 10th or 11th dorsal spine (Ref. 89864). Flank scales large, ctenoid with abrupt transition to small scales on breast; cheek with 3-5 rows of small scales; caudal fin with tiny scales to margin; no scales on other fins (Ref. 89864).

Colouration: Breeding males: head below eye dark blue/black with light-blue interorbital band, nape and postorbital part of head light blue to gray; body blue/dark blue with solid light-blue midlateral stripe and less well-defined light-blue dorso-lateral stripe; anteriormost 2-5 vertical bars darker and superimposed on pattern of horizontal stripes; vertical bars interrupted where intersecting light-blue horizontal stripes; caudal peduncle dark blue/gray with light-blue midlateral stripe extending onto caudal fin; belly and breast dark blue/black; dorsal fin light blue with black submarginal band, in some males submarginal band not solid, and light-blue lappets; caudal fin with dark-blue/black membranes, clear posterior margin, and light-blue rays; anal fin blue with broad black submarginal band, light-blue anterior margin, and 2-10 small orange/yellow ocelli on posterior part of soft-rayed portion; pelvic fin black with light-blue anterior margin; pectoral fin with dark-gray/black rays and clear membranes (Ref. 89864). Females: head gray/green with green iridescent spots on post-orbital part and black opercular spot; dark interorbital bar with some green iridescent spots; short, dark horizontal stripe behind eye not extending to opercular spot; body gray/green-yellow with solid black midlateral stripe extending to but not onto caudal peduncle, and interrupted black dorso-lateral stripe; faint dark vertical bars over horizontal stripes, more conspicuous in mouthbrooding females; dorsal and anal fin gray with faint black submarginal band; caudal fin with black spot at base; membranes light gray with dark submarginal band and outer margin clear; rays light-blue/white; pelvic fin gray with narrow white/light-blue anterior margin; pectoral fin clear with gray rays (Ref. 89864).