The dump command does not block activity in the database, though it could degrade performance if your disk subsystem and/or cpu are currently running at/near max capacity.
The reorg and dbcc commands may or may not block user activity ... it depends on which of the reorg and dbcc commands you're running. It will also depend on your ASE version (eg, SP100 has new capabilities that allow some older blocking-type commands to run without blocking).
As for having seen blocks in the past ... we'll need a bit more detail (eg, actual/complete command that was doing the blocking, what commands/user-activity was being blocked) if you're looking for explanations.