I'm imagining a knife. Not very sharp = dull, right? Is that the origin of the use of "obtuse" to mean "not very bright"?
English has long associated terms for wit and weapons, beginning with sharp in Old English:
Old English scearp “having a cutting edge; pointed; intellectually acute, active, shrewd; keen (of senses); severe; biting, bitter (of tastes),” from Proto-Germanic *skarpaz, literally “cutting”. . . . The figurative meaning “acute or penetrating in intellect or perception” was in Old English. . . .
It appears that a similar figurative usage influenced the meaning of obtuse (both in English and its source languages):
early 15c., “dull, blunted,” from Middle French obtus (fem. obtuse), from Latin obtusus “blunted, dull,” also used figuratively. . . . Sense of “stupid” is first found c.1500.
Conversely, dull “stupid” (from Old English dol “dull-witted, foolish”), blunt (perhaps related to blunder), and keen “clever, wise” (from Proto-Germanic *kan-) developed in the other direction, referring first to wits c.1200 and later becoming associated with weapons and tools in the 13–14c.
Thus, while these words are unrelated etymologically, the wit/weapon metaphor has influenced them all to describe both physical and mental edges in English.