Background
On many WMF wikis, blocking IPs and named accounts at the same time as a Checkuser is considered unwise, even if no explicit connection is made. If an explicit connection is in fact made, the action would be against the global Privacy Policy. For example, blocking "Foo" with {{checkuserblock-account}} <!-- Foo: sock of Bar --> and the underlying IP 192.168.1.1 with {{checkuserblock-account}} <!-- Foo: sock of Bar --> also.

Specific issue
The blocking interface in Special:CheckUser's "Get Users" interface is a great time-saver for blocking large volumes of socks, however it doesn't have guardrails against blocking IPs and accounts together as described above. The user has to manually check that they've not selected any IPs if they want to mass-block accounts or vice-versa. Of course it is incumbent on advanced permissions holders to be diligent in their work, but we should make doing the right thing as easy as possible.

I suppose the user story would be "As a Checkuser, I want an efficient method to block accounts from the'Get Users' interface that minimises the risk of unintended information disclosure as above". I'm not sure what form that would take, but a confirmation step / warning when blocking IPs and named accounts together in the interface is the best idea I've had.