org-agenda-dim-blocked-tasks

org-agenda-dim-blocked-tasks is a variable defined in `org-agenda.el'.
Its value is

t

Documentation:
Non-nil means dim blocked tasks in the agenda display.
This causes some overhead during agenda construction, but if you
have turned on `org-enforce-todo-dependencies',
`org-enforce-todo-checkbox-dependencies', or any other blocking
mechanism, this will create useful feedback in the agenda.

Instead of t, this variable can also have the value `invisible'.
Then blocked tasks will be invisible and only become visible when
they become unblocked. An exemption to this behavior is when a task is
blocked because of unchecked checkboxes below it. Since checkboxes do
not show up in the agenda views, making this task invisible you remove any
trace from agenda views that there is something to do. Therefore, a task
that is blocked because of checkboxes will never be made invisible, it
will only be dimmed.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.