Is it illegal to partially block the aisle on a passenger airliner?
Cargo cannot block the aisle -
FAR Part 121.285, governing U.S. Air Carrier Operations, states in pertinent part:
FAR 121.285-
(emphasis mine)

(c) Cargo may be carried aft of a bulkhead or divider in any passenger compartment provided the cargo is restrained to the load factors in §25.561(b)(3) and is loaded as follows:
(4) Its location does not restrict access to or use of any required emergency or regular exit, or of the aisle in the passenger compartment.