I believe the first reputed example showing a "seeing-eye dog" being used by a blind person comes from a mural that was discovered and dated to the first century AD (so about 2000 years old - give or take). The mural was uncovered in the buried ruins of the ancient Roman city of Herculaneum. Here is a picture of that ancient Roman mural, reputed to show a poor blind man with a guide dog on a lead.

A 13th century-dated arabesque artwork out of China also has been found, showing a blind man being led by a dog. Here is an image of a reproduction of that artwork.

In terms of actual formalized training of guide dogs to help blind people navigate around, that seems to have originated around the latter half of the 17th century or so in England. There is even an old English alphabetic poem/song (designed to teach kids their ABC's) from that time period that, in some versions, was published as beginning with the verse "A is for Archer, who shot at a frog; B is for the Blind man, led by a dog".

As for "pioneers" in this field of purposefully trained guide dogs for the sight-impaired, that can be traced back to the 1780s in Paris, France, at the “Les Quinze-Vigts” hospital for the blind. That hospital was the first recorded example of a facility that actually began deliberately training and using dogs for the purposes of aiding blind people.

In 1819, a guy who was originally born in Germany but who later moved to and lived in Vienna, Austria by the name of Johann Wilhelm Klein - he was the founder of the "Institute for the Education of the Blind" (Blinden-Erziehungs-Institut) and recognized as a pioneer for the education and aid for the blind - wrote and published a book titled "A Textbook for Instruction of the Blind", which is typically recognized as the first real descriptive text to mention the training of and use of canines to be employed as an aid to help blind people navigate around.