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Mix-blend-mode and isolation

I try to create an “inverted” flashlight effect with an black circle. (later with mouse-movement)
Therefore the circle is set to mix-blend-mode:darken.
But the circle should not inherit the background and other elements. Only the text underneath.
I found the isolation: isolate css-value, but I can´t get it working.
The effect I would like to achieve:

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