Final answer:
Netflix faced the challenge of Blockbuster and Walmart quickly extending their brands into DVD-by-mail services.
Step-by-step explanation:
One challenge that Netflix faced in the competition with Blockbuster and Walmart was that Blockbuster and Walmart were able to quickly extend their strong brands to also become synonymous with a DVD-by-mail subscription service. This meant that these well-known brands already had established customer bases and trust, which made it harder for Netflix to compete.