All right, time for some serious negotiations!If you're playing Wizgold, which is probably not the case if you're using Dosbox, you can most certainly trade with Blienmeis after enough negotation. There are three stages of NPC likeitude:
Stage 1 - Truce, Fight, Run. The NPC is royally pissed, and wants your head.
Stage 2 - Truce, Spell, Use, Fight, Run, etc... The NPC is ambivalent, and will spill his/her guts and items with a little persuasion.
Stage 3 - Truce becomes Talk. The NPC likes you, and will trade with you.
Every "Pleased" response makes the NPC like you better, i.e. have a good chance to move up a stage. "Angry," of course, gives a chance of moving down, and "Reserved" is neutral. At any time during negotiations, the NPC may decide to leave, or even attack you.
I had a Stage 1 Belcanzor on my hands one time. I gave him gold, charmed him and hit the Peace button. He "looked pleased" then attacked me, because he hated me so much.
In your case, when Blienmeis leaves, you should move out of the room, then walk back in and Peace with him some more. Making good use of saving when pleased, loading when angry, Blienmeis should eventually warm up enough to your party to trade with them.
Good luck!
Chris