Get the identifier first,
curl -v --silent -H "Accept: application/vnd.docker.distribution.manifest.v2+json" -X GET http://localhost:5000/v2/cdl/oa/oa-frontend-pkblu-v1/manifests/master 2>&1 | grep Docker-Content-Digest | awk '{print ($3)}'
Then delete it
curl -v --silent -H "Accept: application/vnd.docker.distribution.manifest.v2+json" -X DELETE http://127.0.0.1:5000/v2/cdl/oa/oa-frontend-pkblu-v1/manifests/sha256:ae7a8a9605becc07c244e476c5430f800adc8f0e7b669fd596975a70bd102e1b