In Blackmon v. State, 642 S.W.2d 499 (Tex. Crim. App. 1982), the court of criminal appeals concluded that "it is too broad a statement to suggest that a witness taking the stand and testifying never waives his privilege against self-incrimination with respect to extraneous activity." Id. at 501. Instead, the witness may invoke the privilege at the "threshold" of "any particular transaction or area of testimony" that is unrelated to the testimony provided on direct examination. Id. at 502.