Is the attire described in the question, consisting of a tunic, a loose blouse, and a non-tight headscarf, or trousers with a blouse reaching the knees, permissible according to Islamic law?

The Shari'ah has not specified a particular garment for a woman's hijab. However, scholars have mentioned conditions for this attire. If these conditions are met in the garment mentioned in the question, then it is permissible; otherwise, it is not.