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"Block field updates" only affects UPDATES to a field that already contains a value. If you select all, a value will still be able to be populated initially from any source.
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While you should tick as many of these boxes as possible (taking an opt-in approach is recommended, though admittedly few Marketo users do this) you may want to allow Flow change data value because that can be your "informed" method of changing values when all other methods are blocked.