The main purpose of the monthly IFS blacklist information sent out via e-mail is to provide guidance for data assimilation purposes.
If a station appears in the IFS blacklist it does not necessarily suggest poor observation quality. Stations may be blacklisted because of limited representativeness relative to the IFS grid scale, or it may be a new station which is being passively monitored for a certain period before being actively used by the IFS data assimilation. Some temporary blacklisting may be associated with the TAC->BUFR transition.
If a station appears in the IFS blacklist this does not necessarily suggest poor observation quality.
New stations may be passively monitored for a certain period before being actively used by the IFS data assimilation.
ECMWF may not use an observation because (for example) it is in a fjord not resolved by
the ECMWF model, whereas this would be usable in a higher resolution model.
There are also some complications due to the TAC to BUFR migration, see the
overview at https://confluence.ecmwf.int/display/TCBUF/ .
...
ECMWF aims to implement IFS blacklist changes in the second week of each month. During the first week, analysts evaluate automated and manual proposals for changes and run model experiments to test the blacklist change. Operational change requests are then submitted in the second week of the month (typically Monday or Tuesday). Occasionally operational IT constraints mean the change is postponed by a few days.
...