incorrect capital gains calculation

The blended tax rate has nothing to do with how your tax is calculated. It's an essentially meaningless number. You should ignore it. It is not used to calculate the tax on capital gains or on anything else.

The blended tax rate is an attempt to give you an idea of the overall percentage of your income that you pay in tax. The basic idea is that it's the tax divided by income, expressed as a percentage. But exactly what taxes are included and what income is included is not clearly defined. Some people have posted explanations of how the blended tax rate is calculated, but they don't all say the same thing.