A rule question came up in game last night about the interaction of A Noble Cause and Fortified Position.

Does Fortified Position or Nightmares blank renown on the character completely even if it is gained from another source or does it only blank it on characters that only got it from Noble cause? e.g. Brightwater Man-At-Arms gains renown from A Noble Cause. It is blanked by Fortified Position, while Core Brienne of Tarth has renown and then has renown added again by A Noble Cause, thus -Renown + Renown equals Brienne having renown.

I believe I may have answered this. Both say printed text box so I would think A Noble Cause is adding above the printed text box.

Correct. Blanking effects only "take out" the effect of the ink on the card. If an effect or text is gained from a different source, it cannot be affected by blanking.