Mr Blessed is the voice of one of three talking toilets in the Pyramid stage field to highlight the 2.3bn people who have no access to a basic toilet.
Actor Kathy Burke and musician Cerys Matthews will also voice "babbling bogs" to promote WaterAid's Toilets Save Lives campaign.
Around 175,000 people are expected to attend the festival from Wednesday.
WaterAid's Marcus Missen said: "Who hasn't wanted to listen to Brian Blessed while sitting on the bog?"
Billed as the "swankiest" and "best seats in the house", the talking toilets are among more than 5,000 loos being installed across Worthy Farm ahead of this year's festival.
They are being used by the stars to share the "huge problems of not having a toilet".
One of the talking toilets also has a "daunting two-way door" which appears see-through to the person inside but as a mirror to those on the outside.
A spokeswoman for WaterAid said it was designed to make the user feel "exposed" to highlight the one in three around the world who have "nowhere safe to go to the toilet".
"As they use the loo with a view, they'll have an epic view out to the Pyramid stage crowd," she said.
"Yet they'll also have to grapple with feeling visible to all outside, with their trousers down, whilst those waiting check their reflection."
Gates to the festival site open from 8:00 BST on Wednesday.
Acts scheduled to perform include Adele, Ellie Goulding, ZZ Top, Disclosure, Wolf Alice, Tame Impala, Beck, Stormzy, Madness and Foals.
Actor Brian Blessed will be joining music lovers on the toilet at this year's Glastonbury Festival.