When a particular LORAN-C station is part of multiple chains, it will invariably come in the situation where it is supposed to transmit two pulses at the same time. This is not technically feasible so therefore only one of the pulses will be transmitted and the other will be "blanked".
There are various mitigation rules deployed, in some cases one of the chains always wins, this is called "priority blanking" and is generally used if one of the signals is a master signal. Another is "alternate" where the signals take turn in winning the race.
The one thing I had not been able to find anywhere is how long time around the winning signal would be protected by the blanking, so I decided to find out.
The transmitter on Sylt is both 7499M and 6731Z and uses priority blanking in favour of 7499M, and since this is also the transmitter closest to me all I had to do was to capture the max value over the 7499 GRI and plot it.
As far as I can tell it is the usual design rules which govern blanking: 990usec ahead and 10900usec after the t0 point of the master signal.