Real-time Block Lists (RBLs) are databases of IP addresses and domain names which are considered confirmed sources of spam.
RBLs are individually operated by a collection of businesses and non-profit organisations. They aren’t actually responsible for blocking email, email providers must incorporate RBLs into their filtering tests in order for email from a listed IP address or domain to be blocked. Each RBL operator has their own criteria for adding and removing entries from their database.
When email providers detect a large number of delivery attempts to non-existent addresses they often report the sender to an RBL operator.
Some RBL operators have special email addresses which add any IP or domain which attempts to deliver email to them to the block list.
The addresses are not used to subscribe to newsletters etc. so the only way they could end up on a list is through website scraping or some other such activity.
Large email providers are also know to utilise Spam Traps.
A URL blacklist lists the URLs referenced in the body of spam emails. If your website is listed on a URL blacklist then there is a good change that it has been compromised and is part of a spam campaign.
How does this affect deliverability?
Badly. Presence on an RBL will impact deliverability to a significant degree while presence on a URL blacklist will likely kill deliverability completely.