The blocking period is the duration of a task that is over 50ms. For example, a task that is 120ms in duration will be calculated as 70 ms of blocking time.
In Calibre, Total blocking time is calculated by Lighthouse, which is part of our testing suite.
Pages with < 300 ms of blocking time can be considered as fast, 300—600 ms as average where as durations over 600 ms are considered slow.
For more information about total blocking time, read the guide on web.dev