Blockbuster and WalMart fulfill different roles. WalMart is your hypermarket i.e. rather like CarrreFour here in Malaysia. It is therefore ubiquitous and that explains why it sells so many DVDs. It's a huge retailer with multiple stores.

Blockbuster on the other hand deals only with DVD/recordable media. It is both retail and rental. I doubt very much it will disappear anytime soon.