Fab find - I love Jean Varon pieces. Particularly the huge bishop sleeves which your blouse seems to have.
The Bath museum has a maxi dress in the same fabric of similar design but with a pointed collar dated c. 1973 , so the first thing I would ask is that you check the hem to check it has not been shortened or altered. (compare the thread colour, and stitching style to other areas on the garment)
http://www.fashionmuseum.co.uk/collections/collection_search/SearchDetails.aspx
The Jean Varon label often had different garments running variations on the same fabric or style and I have seen seperate blouses so it is possible it was always a blouse.
The 1960's garments have less care information labelled (usually just dry clean only, or nothing at all!) as the requirements in law to provide this information came into force in 1970, some manufacturers began to include this information toward the end of the 60's but not to the extent you see afterwards.
I see your label has another attached at the bottom, can you show this?
As for rarity - nearly all vintage garments of a certain age is 'rare' to some extent and you would be hard pushed to find exactly the same item again in anything. Those in their original condition are rarer as people altered, mended and adapted pieces over the decades to fit new fashion, shapes and trends.
