I've not seen anything about using the dried blossoms to make a tea and those I know that use them to make wine put them in way before they are dried. While I've not tried hard the few times I have set aside a Dandelion blossom to dry it still set seed. Every reference I see to making tea uses the leaves for that purpose.

Usually fresh dandelion blossoms are used to make tea. I don't think dried would hurt you, but they might not taste all that good. Visit www.eattheweeds.com