Learn French: Mon blouson est dans l'armoire. - My jacket is in the wardrobe.

A blouson is a short, casual jacket that often gathers at the waist, like a bomber jacket or a leather jacket. A veste is usually a lighter jacket or a tailored blazer, while a manteau is a heavy, longer winter coat.

Is armoire masculine or feminine, since it uses l'?

It is feminine (une armoire). Nouns ending in -oire are very often feminine in French. Because the word starts with a vowel, we must use the contracted article l' instead of la.

Could I use placard instead of armoire for wardrobe?

You could, but they represent slightly different things. An armoire is a standalone, movable piece of furniture. A placard is a built-in closet or cupboard that is part of the wall.

Why do we use dans instead of à here?

We use dans because it specifically means physically inside of a three-dimensional space or container. Since the jacket is enclosed inside the wardrobe, dans is the perfect fit.