Definition
A person who does not support your product and can block the sale.
See The most important buying roles
A Blocker may support an alternative product or have requirements which are not compatible with your product.
For example, a person in a similar position to your Champion may be the Champion of a competitor. The CIO may support a policy not to use cloud providers.
It is importarnt to identify blockers early enough and come up with a strategy to overcome their objections.