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Michelle, I can not speak to the Black Belt series other than I had reviewed sometime in the past and did not pursue.
I have been in the PowerBI space for sometime and have landed on 3 places to my self training.
1. sqlbi.com has some of the best materials - I am taking Mastering DAX and refer to their videos regularly2. Enterprisedna.co - I am a member and have taken multiple trainings3. https://www.skillwave.training/ - I have yet to take the course work, However, I will be when time permits I will be taking the PowerQuery AcademyMy recommendation would be Enterprisedna.co - this has very practical training with good examples - Sam McKay has quite a few Financial and Budget related materials which you do not find often. (Mainly Retail oriented examples - (our Credit Union data is very different but many principles are the same).
Hope this helps.