Blade servers can be in clusters, yes, however just because you
have blade servers does not mean that you necessarily have a
cluster.
A cluster is a group of servers (or guests) using the same
resource pool, while the hosts in the cluster try to load balance
all the guests in which they have.
A blade server is a physical device that houses multiple
thin-devices which are the actual servers which then house guests.
If multiple guests aren't sharing the same blade server then
technically you don't have a cluster.