A blocked message by itself does not mean anything is wrong. The processing thread is simply reporting that it can't continue processing the given source results since they have not been delivered yet. After results are delivered, which is happening in a connector worker thread, then processing can resume.
> I see that materialized views are loaded but i couldn't figure out anything about the source queries which were getting blocked.
It's perfectly normal for processing to block on a source query as there are two threads involved. The processing thread when it has to wait for a connector thread will report in the log that it is blocking, but will resume once the connector has delivered the results.
If the issue seems specific to 8.7, I will resolve it as out of date.