Why is the Blended Tax Rate showing as 10.1% when the taxable income is well below the $11,000 threshold to be taxed at 10%?

That difference is inconsequential, and for you is the result of the tax tables being used....where a single tax is assessed for an entire $50 region of taxable income.

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For example, a single person with 3058 of taxable ordinary income, from the IRS tax tables....any taxable income from 3050 to 3100 is assessed a $308 tax.

So your tax is 100x($308/3058) =  10.07% which rounds to 10.1%

Now, if you had $3099 of taxable income (at the top of that particular table range)

...then the tax would be 100x($308/3099) = 9.9%

All because the same $308 tax is assessed over a $50 range of Taxable incomes in the table.

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In the future, as you start getting more income from investments...including dividends and capital gains, there may be higher deviations from the exact tax bracket you are in for the "blended" rate, since dividends and Capital gains are often taxed at differentiates...and the "Blended" rate is really just a handwaving value of no exact significance.....other than to let you know at what approximate rate you are being taxed.