Yet without knowledge, the translator will eventually be found out to be working in the dark. It might take a while, but it will probably happen. This suggestion might only have validity if your client is not an agent. Few agents have any need or desire to learn the fields in which their translators work. For the translator working with a direct client, however, knowledge of the field can mean the difference between having happy clients and having no clients.
Trade journals are an excellent place to start.
They are not as heavy in content as scholarly journals, and will also provide exposure to some of the business aspects of your client's industry. This is a very personal question, of course. There are some translators who translate because they are capable of translating and because it pays the bills, regardless of their hatred of the subjects they are forced to translate, or perhaps even of the very activity of translation itself. My personal approach would be to do something else, but that is difficult for me to say, because I like translation and have knowledge and interest in the subjects in which I translate. A rhetorical question, of course.