Is it bad practice to use the DTOs of the system under test in the actual test?

I am creating some "black-box" functional tests for a Java/Spring web application that has many DTOs, some are quite complex. When it comes to create the JSON body for the test HTTP requests, it would save me a lot of time if I were to use the existing DTOs, however doesn't it defeat the "black-box" part? If I'm not supposed to use them, what is the simplest approach?

Thank you for your attention.