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Data blending has many limitations, including the behavior you're seeing in your new worksheet. AKA the "Cannot blend the secondary data source because one or more fields use an unsupported aggregation" error that you see when you mouse over AGG(Crime Count) on the last Marks card.
Because data blending is an ad-hoc LEFT join that aggregates data from the secondary data source, Tableau cannot always perform aggregations that require the full domain. For example, COUNTD(). See Troubleshoot Data Blending - Tableau for more information on this and other errors that may occur while using data blending.
You can still use the field with COUNTD() from the secondary data source if you remove all dimensions from the secondary data source (i.e. remove the [Date Occurred] filter).
Better yet, you can use SUM(Number of Records) instead of COUNTD([DR Number]) because that data source appears to only have one record per [Dr Number].
Typically I recommend joining data rather than blending it. If your data sources are from different databases, you can use a cross-database join. See Join Your Data: Combine tables from different databases for instructions.
Hope this answers your question!