Hi Viktoria, It can be used in a positive way depending on the connotation you want to put in the whole sentence or paragraph. If you think of "unspeakable" as shocked, you can be shocked if you see a street performer doing something you have never seen before. You were surprised, or shocked, or had that unspeakable feeling (more speechless in this case) for what you were seeing. That can be highly positive if you are writing a creative text. (for example: "I have never seen that before. My feelings were unspeakable." Though, I wouldn't use it on a more technical paper/letter as "unspeakable" can be a subjective word and the reader may not fully understand what you are trying to say. Have a wonderful day.

In the negative, the idea is that something is so bad that it would seem wrong or disgusting to give the details - e.g, ‘the unspeakable actions of the serial killer’ In the positive, the word is an intensifier with a different idea that there just aren’t words capable of describing something. From the top of the mountain I looked out upon a landscape of unspeakable beauty. (It was very beautiful) There are a number of similar expressions such as ‘indescribable’ , ‘beyond words’ which personally I would prefer to ‘unspeakable’, but ‘unspeakable’ isn’t wrong here.

If you describe something as unspeakable, you are emphasizing that it is extremely unpleasant. By definition it cannot be used in a positive connotation but then again, a writer can always play with words.

Depending how you use it, yes, but you are right in your suggestion that it tends to have negative connotations, especially if used on its own. Unspeakable kindness, would mean kindness at a level beyond words, for a positive example.

You could probably say "unspeakable joy".