No it is not possible by using a sitemap. Very few aspects of a sitemap are dynamic. Mappings in the sitemap are static. @rossko57’s advice to use [visibility=...] is an option if you only have two or three discrete mapping patterns that you anticipate you will want. If you want to have full, arbitrary control of the mapping levels you’ll need an intermediary item on the sitemap and a rule that translates the the sitemap item to your myblinds item.
Sitemap:
Switch someSitemapItem [0="up", 1="half", 2="open", 3="down"]
Setpoint item=blindsHalfLevel
Then your rules files should contain some logic such as: When someSitemapItem changes,
if someSitemapItem state is 1 (i.e. “half”) set myblinds to blindsHalfLevel state.