Quick answer

Most fake Blancpain complications split into two groups. The module-based ones actually work, and the decorative ones only look like they work. A complete calendar with moonphase, a standard chronograph, and a basic tourbillon can all be built to function at replica prices. A true perpetual calendar, a minute repeater, and an equation of time cannot. If a seller says otherwise, the movement will correct them the first time you set the watch.

What a complication really is on a replica

On a genuine Blancpain, complications come from calibers built and finished in Le Brassus. On a replica, they come from a base movement plus a module stacked on top. The base is usually a Miyota 9015, a Seagull ST2130, an ETA 2824 clone, or a Chinese 7750 in chronographs. The complication lives in the module, and the module is where the budget went or did not. I check the base first, because a good module on a weak base still keeps bad time.

Complications that hold up

Complete calendar with moonphase. This is what most people mean when they look at fake Blancpain complications. The Villeret Quantième Complet layout, with day and month apertures, a date hand circling the dial, and a moon at 6 o'clock, is well within reach. The architecture even matches: Blancpain's own complete calendar is a module on a base movement, so a replica copying that layout is not faking the concept, only the execution. A working version needs a 59-tooth moon disc and a month wheel that accounts for 30 and 31 day months. Better mid-tier examples get this right.

Chronograph. Blancpain's flyback chronographs use a column wheel. Replica versions mostly use a 7750 with a modified lever setup. Start, stop, and reset all work. The flyback usually does not, meaning one push will not return a running chronograph to zero and restart it. You still get a usable stopwatch, just not the full function on the dial.

Tourbillon. The Seagull ST8000 family gives you a real rotating cage at one revolution per minute. That is a genuine tourbillon in the strict sense, not a decoration, and you can watch it move through a display back. Rate stability and cage finishing are nowhere near a Blancpain tourbillon, and nobody is pretending otherwise once the caseback is off.

Complications that do not hold up

• Perpetual calendar. The whole point is mechanical memory of leap years. Replica perpetual calendars are almost always complete calendars with a 31-day wheel and no leap-year cam, so they need correcting in March, not in 2100.
• Minute repeater. Two hammers strike gongs for hours, quarters, and minutes. Functional repeater replicas do not exist at consumer prices. What arrives is a slider that moves nothing or a gong you cannot hear.
• Equation of time. The cam that tracks the sun's offset from mean time, up to about 15 minutes. No replica implements it.
• Carrousel. Blancpain's Carrousel Volant Une Minute is a carrousel, not a tourbillon, and the difference matters to collectors. A rep sold as a carrousel is typically a tourbillon with different engraving on the bridge.

How to verify a moonphase before you buy

Ask for a short video of the moon disc advancing. A real moonphase steps once per day and finishes a cycle in about 29.5 days. Plenty of cheap watches use a 24-hour day and night disc instead, which flips between sun and moon every 12 hours and looks wrong the moment the date is set. Check the aperture too. The Villeret moonphase window is a shallow arc; rectangular or oversized openings are a giveaway.

Correctors, pushers, and the caseband

Genuine Villeret calendar watches adjust through recessed correctors, which keeps the case side clean. Replicas often move those correctors to the caseband as visible pins, or ship a plastic pusher and a caseback with a flat spot for the tool. Look closely at the side profile in photos. Extra pins you can see from across the room are a shortcut, not a feature.

Movement check

Display backs make this the easy part. Look for rotor engraving, perlage and Geneva stripes rather than printed patterns, jewel count, and the beat rate. Most Blancpain-based movements run at 28,800 vph, so a slow 21,600 sweep on a watch claiming a high-end caliber is a mismatch. Set the date forward past midnight and watch how it changes. Instant snap and a slow crawl both exist in real watchmaking, but the behavior should stay consistent across a full month.

Bottom line

Buy the complication you can test in ten seconds. Calendar with moonphase, chronograph, and tourbillon are all verifiable with a video and a loupe. Perpetual calendar, repeater, and equation of time should be treated as cosmetic until proven otherwise. That one rule saves more money than any spec sheet a seller hands you.