The query your tried is just grouping by _time to no Web.dest field is available to filter. Try like this
| tstats count(Web.dest) AS destCount FROM datamodel=Web WHERE Web.action=blocked GROUPBY Web.dest _time span=1m | regex Web.dest="\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$"
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