If he has no plans on building sites/content on any of these to direct traffic to his main site, is there any benefit whatsoever of just reserving domain names and having them redirect to a site without having any content on them? Seems to me the only benefit would be if someone wanted to buy the domain off you.
The main advantage of those can be for general marketing purposes. It might not be so easy to promote johndoelaw, but if you can advertise mytowninjurylawyer in local media, you are more likely to get traffic from people responding to the adverts.
Could this be interpreted as black hat by search engines, or would the additional domains need to be built up and stuffed with keywords and made to appear as informational sites with the sole intention being to drive traffic to another domain?
Unlikely - it’s a pretty standard marketing technique that has been going strong for years. It isn’t as though you’re treating search engines any differently to real people. There could be many legitimate reasons why you’re redirecting from one domain to another - I’ve not heard of Google penalising sites for it.