I have the same (or similar) thing with my watch post-update. Black screen but the green light beneath is still flashing.
The watch is still recording *something* because the app says I have done 187 steps today (although my phone says I have done 1500) and the app registers the battery at 11% currently, but the screen is forever black.
I appreciate FitBit's support offering me a replacement, but they really should be doing things for people outside of warranty too as a goodwill gesture, its quite obviously a fault with the update.
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