Generally, no, you are not supposed to keep the blankets provided by Hawaiian Airlines (or most other commercial airlines). These items are considered company property and are collected at the end of the flight to be professionally laundered and repackaged for the next set of passengers. Taking them is technically considered theft of airline property. This rule applies to both the plush duvets found in First Class and the standard blankets in the "Main Cabin." However, Hawaiian Airlines does occasionally sell "Pau Hana" comfort kits or branded blankets through their on-board gift cart or online store if you want a souvenir to take home. Items that you can typically keep include the complimentary slippers and eye masks found in the amenity kits of First Class and Extra Comfort seats. If you are unsure, it is always best to ask a flight attendant rather than simply tucking the blanket into your carry-on.