The first thing I check on a black bed is whether the black is actually even. Hold it under a window or step outside with a sample door if you can, because a lot of black finishes go patchy or slightly blue in daylight, especially on wood grain pieces where the stain pools unevenly in the open pores. A good black sits flat and consistent whether you are looking at it under a bright bulb at ten at night or gray morning light, and if the showroom lighting is doing a lot of work to make it look uniform, that is worth noticing before it goes anywhere near your bedroom.
On delivery day, run your hand along the edges and the footboard before anyone starts assembling. Chips show up fast on black because there is no grain or pattern to hide a ding, you just see raw material underneath, pale wood or gray metal, standing out like a scar. Check the hardware too, bolts and brackets on black metal frames are often black coated as well, and cheap coatings rub off onto sheets or scratch down to bare metal at the headboard corners within a few months of leaning on it to read. Open the box, check the slats, check that the finish continues on surfaces you will actually see, including the inside edges of a headboard if it is tall enough to be visible from the doorway.
Measuring a black bed matters more than it does with a lighter one, because dark furniture reads larger than its actual footprint in a room, especially if your walls are a pale color. I have seen people buy the exact dimensions that fit on paper and still feel like the bed swallowed the room once it arrived, because the visual weight of black pulls the eye and makes the whole piece feel closer to the walls than the tape measure suggests. Leave more breathing room than you think you need, especially on the sides where you walk past it daily, and measure your doorways and stairwells for a solid wood frame, since those are heavier and less forgiving to maneuver than a boxed metal one.
What separates a good one from a disappointing one comes down to what is under the color. Solid wood or real metal with a baked or powder coated finish wears like iron and ages with a bit of honest wear at the edges, which on black actually looks fine, almost intentional. Cheap black veneer over particleboard chips at the corners within a year and never looks right again, because there is no depth to the color, just a thin skin over something soft. Check the joints at the headboard and footboard, they should feel solid, not held together with cam locks and hope.
The one mistake to avoid is buying black without checking it in your own room's light first, because what looks sharp and deliberate in a showroom can look flat and heavy once it is sitting against your particular walls at night.