Dull, blunt, obtuse are comparable when they mean the reverse of sharp, keen, and acute.
As used of things, especially of tools, weapons, and instruments, dull refers to either an edge or a point that has lost its sharpness by use.
Blunt refers to an edge or point that through use, nature, or intention, is not sharp or keen.
Obtuse applies to the shape of something whose sides converge at an angle that is broader than a right angle or to a thing terminating in a broad blunt point.
In the extended senses of these words, dull (see also STUPID) is the most widely applicable and the richest as well as the most variable in its connotations. It implies, in general, a lack or the loss of what gives keenness, intensity, or activity.
Blunt (see BLUFF for application to manners and utterances) usually implies a lack of edge or point in the figurative senses of these words.
Often, it refers to a person’s powers of perception or to his sensibilities, which normally should be sharp or keen.
It may also apply to matters (as contrasts, critical judgments, and analyses) normally requiring sharp distinction or differentiation and then imply exceptional conciseness and corresponding loss of fine detail.
Obtuse suggests such bluntness of perception or sensibilities as makes one insensitive to emotions or ideas.