A block pool is a collection of block (on a datanode ???)
hdfs dfsadmin [-deleteBlockPool datanode-host:port blockpoolId [force]]
where:
Snpashot from the Overview of HDFS - Web UI (Namenode UI)
Block Pool ID: BP-1024368518-10.10.6.28-1525360368119
Snapshot from HDFS - Web UI (Namenode UI)