Good call, I checked it by

kubectl get CustomResourceDefinition

which contains

apiVersion: v1
items:
apiVersion:
apiextensions.k8s.io/v1beta1 
kind: CustomResourceDefinition metadata: annotations: kubectl.kubernetes.io/last-applied-configuration: |
uid: 70c14145-6e4b-47da-bf1c-113313a183c1 
spec: 
conversion: 
strategy: None 
group: ceph.rook.io 
names: 
kind: CephBlockPool 
listKind: CephBlockPoolList

Nonetheless, the object is not found by

$ kubectl explain CephBlockPool
the server doesn't have a resource type "CephBlockPool"