Blaine and Sam –will- dress up for Halloween. Duh. They always will, but they make a bro pact, that first Halloween in New York, not to tell each other what their costumes will be.
They have their own curtained-off rooms in the loft, so maintaining a veil of secrecy will not be difficult. Tina and Artie know what the costumes are, but they've spent the past few weeks sucking face, so they're not very aware of their surroundings when they're both home.
Sam has to talk Blaine out of Nightbird, though.
"Dude, Nightbird needs a night off."
"But the candy of the world must be protected, Sam."
Blaine agrees in the end, because all the fun costumes they think they'll see on the New York City streets that night will give him ideas to add to Nightbird's clothing repertoire. Sam wouldn't have minded going as Blonde Chameleon, except that his costume is really a voice thing, and if he stayed quiet you'd have no idea what he was. His costume needs to be more visual.
Then, this awesome idea smacks Sam right between the eyes. With Blaine, it dawns on him slowly, like the growing smile on his face whenever he glances at Sam over the breakfast table.
The Hummelpezberry loft throws the best parties in the world. They've heard about that epic Thanksgiving one. (Whenever Rachel or Kurt mention it they both start singing Turkey Lurkey Time. It gives Sam an earworm until he begs that they all sing something else.) Blaine and Sam bring everything with them to the Halloween party in Bushwick. They keep radio silence. They've tossed the costumes in duffle bags and disappear to change in separate rooms.
Blaine's swiped one of Sam's plaid shirts and it looks too big - of course - but he doesn't tuck it in. The jeans are easy. He's found a blonde wig somewhere and combed it out patiently until the bangs fall into his eyes. Sam may be okay with using Blaine's Chapstick, but Blaine knows that Sam will notice if one of his Chapsticks is missing, so he uses his own. He can't emulate Sam's mouth shape, so he settles for a pout, instead. He's practiced his Sam face in the mirror for a week, in between classes or before dinner. Cooper taught him how to point, and while he won't actually point when he brings the truth of Sam to the party, he sees that both Cooper and Sam have that habit of drawing attention. It's a good acting exercise. He also practices "Dude!" and "Fact" and those body rolls. The White Chocolate trademark body rolls are actually very, very easy. Kurt, for one, appreciates watching Blaine practice Sam's stripper moves. Blaine and Kurt always end up fooling around after practice.
Sam's Blaine costume is the same one that he wore at Regionals this past year, except that he switches out the purple tie for a gaudy blue and white spotted bow tie he found at Goodwill while shopping with Tina. The gel part is easy. Just sneak into the bathroom and that's that. Sam makes sure to put back that strategically placed dark hair so that he doesn't figure out why the the level of the hair gel keeps on decreasing so fast. Blaine never does notice. Sam also practices how to be composed and enunciate all his words properly. Good thing he's Blonde Chameleon, so getting the voice right is a snap. Tina helps him make a list of smart words to memorize, just so that he can throw them into conversation, but he promptly forgets them as soon as they all leave.
When Blaine and Sam pop out of the rooms at the same time, everyone falls about laughing, except Santana. Santana knew perfectly well as soon as they'd made that ridiculous pact what each of them would do. She's very vocal about how she figured it out first, although "those aren't even good costumes."
The idea of their bro switch is cute, but Santana would die before she'd admit to it. She doesn't. She's secretly glad that Blaine living in Sam's body and Sam living in Blaine's body isn't a real thing, because that would be creepy as fuck. She hopes that Brittany, far away in brainy MIT, will never figure it out.