bli neder

does bli neder need to be actually said out loud to the other person or is it enough to just think it to oneself even in response to another person?

shalom uvracha, If a person says that he'll do a mitzva it automatically becomes a neder (see nedarim 8), and therefore, he must say 'bli neder' (meaning 'without vowing') out loud, while taking on this commitment (see Ritva: Nedarim, 23:b and Kol Nidrei: 81:10 if it needs to be said in front of other people or may be said out loud just to oneself). However, if a person said Kol Nidrei or Hatarat Nedarim before Rosh Hoshanah then even if he didn't say bli neder there is no neder (vow) since he already canceled his vows made. We rely on the Hatarat Nedarim for cancelling the types of nedarim that the person just said that he'll do a mitzva, but didn't say a vow explicitly, (see Minchat Shlomo, 1:91). Even so it is still a good habbit to say bli neder whenever mentioning that we will do a mitzva. all the best!