It is counted as a 2021 event, my friend. It's not an opinion, it's a fact. Blast Global Finals was the 1st event of 2021, which is anyway obvious because HLTV Top20 of 2020 was already out and over before the tournament finished.
In addition, you can go to news section and search January 2021 news. The tournament news page officially marks Blast Premier Global Final as "The first big event of 2021".