let me give it a try. question, did you add both below in the app setting for AzureWebJobsStorage?

"AzureWebJobsStorage__accountName": "rgbeta18230",
"AzureWebJobsStorage__credential": " managedidentity",

I have created the exact func app based on your description, blob trigger in storage account rgbeta18230blob and AzureWebJobs in rgbeta18230.

test1: using AzureWebJobsStorage with connection string, trigger works.

test2: removed AzureWebJobsStorage and added "AzureWebJobsStorage__accountName": "rgbeta18230" only, getting the same error as yours:

test3: further added "AzureWebJobsStorage__credential": " managedidentity", trigger works again.

in short, AzureWebJobsStorage__credential is needed to tell the FA to use managedidentity. leaving it out will not use managedidentity. Please give it a try.