A speaker gets dressed in their finest black, and the poem lingers over the fabric: black trimmed with more black, silk on velvet. The outfit sounds rich and formal, the sort of thing you would wear to a funeral or some other solemn occasion. The poem never says what the occasion is, so we are left to guess. Mourning seems the most natural reading, though that is an inference from the clothes rather than anything stated.
Then the poem turns on the word "but." The speaker wishes they could have found their heart, which they call a "blessed crumb." The phrase is oddly tender. The heart is tiny, almost nothing, and also holy. It suggests someone so emptied out by loss, or by the pressure to perform feeling, that they can dress perfectly and still not locate the emotion that belongs inside the clothes.
The mood is quiet, dry and sad. The elaborate outfit is all surface, and the lost heart is what's missing underneath.