The phrase "opaque envelopes" refers to envelopes that are not see-through or transparent. They cannot be easily looked through, making the contents inside hidden or secret. Full definition
The treatment assignment sequence was masked to the investigator with the use of opaque envelopes. (academic.oup.com)
low risk of bias (e.g. telephone or central randomisation; consecutively numbered sealed opaque envelopes); (cochranelibrary-wiley.com)
Three studies were judged unclear risk of bias: Rowley 1995 and Allen 2013 gave no information about the process of random allocation; and Flint 1989 used sealed opaque envelopes but did not specify any numbering. (cochranelibrary-wiley.com)