Astral prevents your item from going to 0, it goes to +4 instead when you fail to 'up' your item.
Astral is one time use. Whenever you fail, it deducts one. Only 1 astral at at time is needed.
It does add a blue stat (Astral 1)
You do need immortal to add astral.
No, astral can't be exchanged to prevent people from easily selling silks for gold.
Notice also that after +5, you should add steady because if you fail, you may lose durability also.