Final answer:
A sympathetic antagonist is the type of chemical that blocks secretion of adrenal catecholamines.
Step-by-step explanation:
The type of chemical that blocks secretion of adrenal catecholamines, such as epinephrine and norepinephrine, is a sympathetic antagonist. Sympathetic antagonists work by blocking the adrenergic receptors, which are the receptors to which these catecholamines bind. This blocks the sympathetic influence on the target organ and cancels their effects.