Requesting refunds can be the reason for some vendors to blacklist you, as refunds negatively impact statistics and some buyers will unscrupulously request refunds on most of their purchases.
The way blacklisting works is each vendor can set a certain threshold. So, if a buyer has been blacklisted by, say, 3 vendors (many vendors will automatically add buyers to the blacklist if they request a refund), then any other vendor who has set a threshold of 3 will have automatically blocked that buyer. With a threshold of 4, 4 vendors will have had to blacklist a buyer, etc. This is most likely cause of this if you have not been blocked specifically by the vendor.
This basically is the same as a store posting a sign on their door that says, "We Reserve The Right to Refuse Service, at any time for any reason" Therefore there is nothing we can do to prevent you from being added to a blacklist. This is something that each vendor determines on their own.