For such a short poem, "Tell all the truth but tell it slant —" contains a lot of consonance. Note how often the poem turns to the /l/ sound, for example, especially in the first line:
Tell all the truth but tell it slant —
The first line also contains consonance (and alliteration) of the /t/ sound in "Tell," "truth," and "slant." Taken together, this consonance makes the first line feel memorable and assertive.
Later, in line 4, consonance appears with the /p/ sound and the sibilant /s/ sound:
The Truth's superb surprise
These sounds connect the words "superb" and "surprise," emphasizing the idea that the truth often contains wonderful or beautiful surprises—even if human beings aren't always able to register or make sense of this beauty. The consonance and alliteration in this moment also just makes the line sound more musical and satisfying. The speaker's language reflects the beauty that often lurks within the truth.
The poem's last moment of consonance comes at the very end of the final line, when the speaker alliterates the bold /b/ sound, saying, "Or every man be blind." The immediate repetition of the /b/ sound gives this moment a heightened and rhythmic feeling, ensuring that the poem ends on a strong and decisive note. This assertive tone reveals the speaker's confidence in the belief that overwhelming people with knowledge simply makes it impossible for them to see the truth at all.