Let the boy try along this bayonet-blade How cold steel is, and keen with hunger of blood;
The poem opens with the speaker telling who-knows-who to let the "boy try along" the blade of a bayonet (a rifle with a knife attached to the end of it).
Try what? It turns out the speaker wants the boy to feel how cold the steel of the blade is, and realize how hungry it is for blood.
The word "try" means something like "try out" or "test the advantages of." The speaker seems to be saying "let the boy try his bayonet out" so he can understand just how bloodthirsty the thing is.
Wait. We get that bayonets are for killing, but can a blade actually be bloodthirsty? Not literally. But the speaker's using figurative language here to make the bayonet seem like it has will power—that its need to kill goes beyond the boy's use of it as a weapon.
The boy is probably a soldier, so why does he need to learn about his bayonet-blade? Perhaps he's just a young kid, not used to guns and killing. Maybe he's only in training, just now learning the tricks of the warfare trade.
Blue with all malice, like a madman's flash; And thinly drawn with famishing for flesh.
The speaker continues talking about why the boy should "try along" the blade. It is so he can also see how it is "blue with malice" and "thinly drawn with famishing for flesh."
"Blue" makes us think of somebody that is blue (i.e., deprived of oxygen, dead, cold, lifeless).
But the word "blue" can also mean "livid with excitement," a meaning that makes us think of "keen" in the previous line.
The word "flash" here can mean a few things. It can refer to an outburst of light, like the one from a rifle when it's fired.
But a "flash" is also an insignia. During World War I, many soldiers suffered from shell shock, a psychological condition resulting from the stresses of battle and characterized by fatigue, an inability to prioritize things, blindness, limping, and a generally "dazed" and confused disposition. Soldiers suffering from shell shock had to wear a blue insignia or "flash" to indicate to others their status and potential for unpredictable behavior. Yep, that's as awkward as it sounds.
The phrase "thinly drawn" literally means that the blade is "thinly stretched out," like a skeleton, skinny because it is starving and pining ("famishing") for the flesh it hasn't eaten in a while. Now that's a gross metaphor if we've ever seen one.
So what's the gist here? The speaker wants the boy—whoever he is—to see how bloodthirsty and malicious this weapon is. He wants that boy to understand that this weapon is a dangerous, evil object. So evil, in fact, that he personifies it just to show that it's practically got a mind of its own.
Okay, now that we've got one stanza under our belts, it's time to talk form.
Each of these lines has about ten syllables, give or take, which is a sure sign that this sucker's in iambic pentameter.
But there's also some rhyming going on… sort of. After all, blood and blade do sound an awful lot like each other, as do flash and flesh, even if they're not perfect rhymes in the strictest sense of the word. In fact, we call these slant rhymes.
When you have rhyming pairs like this, written in iambic pentameter, you're reading heroic couplets.