Two meanings sit inside the title. "First blood" is the phrase for the opening wound in a fight, and it is also a plausible name for a girl's first period. The poem never says which it means, though "blood in the night" followed by "they say nothing about it to her" points toward the second. Nothing in the text names the girl's age or what exactly has happened. The poem works by withholding, and the reader is left in the same position as the girl: aware that something has happened and given no explanation.
That silence is the poem's central idea. The first line is the only moment of plain, physical fact, and the word "blood" is set down hard as the opening word. After that the poem muffles itself. "They" are never identified, only a group that "take dinner", a phrase with a formal, polite ring, as if the meal were a ritual with no room for what happened overnight. "Her" is the last word of the second line, left alone at the edge of the sentence, and the placement feels like the treatment she receives.
The room is described as "beset / with delicate pictures". "Beset" normally belongs to sieges and troubles, so the decorative pictures seem to surround the diners almost threateningly. The prettiness is a kind of pressure, a politeness so thick that nothing real can be said inside it. Then the pictures take over. The horses are "painted / like shelves for war", and the image is strange and slightly resistant. A shelf is flat and built to carry weight, so the horses look like surfaces made to bear loads, and "painted" suggests war paint as well as the paint of a canvas. The line break after "painted" briefly lets the word stand alone, so we wonder whether we are being told how the horses look or what has been done to them. One possible reading, which is an inference and not something the poem states, is that the girl is being quietly assigned a role as someone who will carry and bear things, with nobody asking her.
The grammar helps this transition. After the first couplet the poem becomes one long sentence. Its verbs, "take dinner" and then "pass", belong to different worlds, but the syntax joins them, so the table and the painting seem to run together. By the end the horses "pass with great calmness / into the devil's wheat", and the dining room has dissolved into a field. "Great calmness" is the unsettling note here. These horses are decked for war and enter an unholy crop without any alarm, just as the family eats without alarm. The calm is the true source of dread, since it suggests that no one will flinch. "Devil's wheat" turns the most domestic of crops, the one that makes bread and therefore dinner, into something tainted. It may faintly recall the biblical image of weeds sown among wheat, but the poem does not insist on this, and the phrase works without it.
The form matches the content. There are four unrhymed couplets, with no capital letters and no punctuation except the apostrophe in "devil's". The short lines, of roughly four to six syllables, have a fragmentary, glimpsed quality, like cuts between images. Without full stops, the poem never settles or concludes, which leaves the girl's situation open-ended. The sound is quietly crafted too. The p of "pictures", "painted" and "pass" is repeated like a soft tapping, and the sibilants of "shelves", "calmness" and "devil's" give the second half a hushed, whispering texture. After the hard single beat of "blood", the poem lowers its voice and keeps it low.
What makes the poem significant is how much it gets from so little. It does not describe pain, shame, or fear, and it does not comment on the adults' behaviour. It sets a bleeding in the night beside a polite meal and a procession of calm painted horses, and lets the contrast speak. The double sense of "first blood" widens the scene. A private bodily event is bound up with images of war, which suggests the girl is entering a struggle that nobody will name aloud. For a reader, the pleasure is in noticing what the poem declines to say, and then realizing that this refusal is its subject.