Short answer
A fake Blancpain 50 Fathoms is a watch that copies the case, dial, bezel, and markings of the Blancpain Fifty Fathoms without approval from Blancpain or Swatch Group, the brand's parent company. The spelling "50 Fathoms" shows up in search terms and listing titles. Blancpain writes the name as "Fifty Fathoms." Copies run from quartz models under $100 to automatic models that sell for a few hundred dollars. None carry the Blancpain calibre 1315 or the brand's case and dial finishing.
The genuine watch, in numbers
Blancpain introduced the Fifty Fathoms in 1953. The name refers to 50 fathoms, equal to 91.44 meters. The 1953 model had a rotating bezel, a screw-down caseback, and automatic winding. Current Fifty Fathoms Automatique references use calibre 1315: three mainspring barrels, 120 hours of power reserve, and a silicon balance spring on recent versions. Steel models list in the five-figure range at US retailers. Most current references have a sapphire caseback, so the movement is visible.
Tells that separate a copy from the real watch
Caseback. A sapphire caseback on a genuine 5015 shows calibre 1315 with beveled bridges and a signed rotor. Copies use a solid caseback, a mineral crystal, or a generic movement with added decoration.
Bezel. The genuine bezel turns with firm detents and lines up with the minute track. Loose rotation and a misaligned insert point to a copy.
Dial print. The Blancpain script is thin and clean. Copies show thick lettering, uneven spacing, and a date window that sits off center.
Lume. Genuine watches use Super-LumiNova in a flat, even layer. Copy lume is thin, blotchy, and dims within minutes.
Price. A steel Fifty Fathoms from an authorized dealer costs five figures. A new watch offered for a few hundred dollars is a copy. No exception.
Serial and reference numbers. Engraving on a genuine case is crisp and matches the papers. Many copies repeat the same serial across batches.
Legal status in the United States
19 U.S.C. § 1526 bars import of goods that carry a counterfeit trademark, and U.S. Customs and Border Protection seizes such shipments. Under 15 U.S.C. § 1117(c), a trademark owner can recover statutory damages of up to $2,000,000 for each counterfeit mark per type of goods when the use is willful. Import of a counterfeit item is a customs violation even when the buyer has no intent to resell. A seized watch is not returned, and a copy has no warranty or service path.
How to check a specific watch
Take the watch to a Blancpain service center or an authorized dealer. Staff check the serial against brand records.
Have a watchmaker open the case. The movement, its finishing, and its engravings settle the question.
Compare the reference number on the caseback with the model it claims to be. A 5015 reference with a quartz movement is a copy.
Request the original invoice from an authorized dealer. Boxes and papers on their own prove nothing.
Blancpain does not publish a public serial number database. Authentication comes from the brand's service center or an authorized dealer, not from a listing photo.