First read the work items records. Add a loop to iterate all the records. Inside the loop Read the Blackout Dates list filtered by Execution Date >= Start Date and Execution Date <= End Date and Stage approved. If record count is one then update the list.
All date must be formatted to yyyy-MM-dd format using the expression. See below example.
use formatDateTime('DateColumn', 'yyyy-MM-dd')
How many records you having?
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