Hypothesis :

Lannister vs Baratheon during joust.

During the challenge phase Lannister attacks Baratheon with a character with deadly. Willis Tyrell defends, does deadly still get blanked ?

Willas Tyrell does not blank anything. He says "While Willas Tyrell is participating in a challenge, participating Lannister and Martell characters do not count their STR." Deadly still applies.