Still having problems with this. The 3m. length of chain I ordered arrived as a 2m. piece and a 1m. piece, despite a note saying I needed a continuous length. But that's a separate issue.
Properly lengthening a blind chain requires that the join be no larger than the chain itself. (A standard clasp won't go through the gear affair at the end of the roller of the blind, on the 4 blinds I have.)
On one of the existing blinds, the loop has such a clasp, so the lengthened blind (for a very tall window) can't be fully raised because the clasp acts as a stop, and if you reposition the chain then the blind can't be fully lowered. Also the bottom of the loop is about 6 feet above the floor, which is inconvenient. And a chain with only 2m. between two clasps wouldn't solve the problem; the window is too large (high).
They do make endless loops of chain, though to fit one might require some roller disassembly, and a standard length might not be suitable.
Easier if there is a way to join the ends invisibly, using something like one of the links from which such a chain is made.
Google shows lots of clasps, but that's not what's needed.
At a guess, there might be a tool which crimps a ball so that it fits fairly tightly around one of the dumbbell-shaped pieces which alternate with the balls on these chains. Or maybe there's a special kind of ball. Or both the above.
How else could they manufacture endless loops?
Does anyone know how this is done / what the parts are called?
In the past, my attempts to repair broken sink chains by just pushing the parts together produced a weak chain which soon came apart, so that's not likely to be much good on a blind chain which has to handle a fair amount of strain. There has to be some better way.