According to the Blackburn Encyclopaedia at Cotton Town, this was...
Built by Ashton Frost of Blackburn, 1897, at 700ft above sea level.
A water storage reservoir standing on the site of a bronze-age burial mound dating from about 1500 BC.
At the end of the 1800s was known as the 'Blackburn Pier'.

Demolished in 2001 as the Borough and North West Water couldn't come to an agreement about its preservation.
The stainless steel disc, which was on a table on the tank indicating the views from the top,
was taken down some time ago [and] is still in the hands of the North West Water Co..

As a secondary school kid, I used to have my lunch on the Tank every Wednesday,
on my way to the school's sports-fields below -- for what I can only describe as 'torture'.
(Otherwise, I have very fond memories of this place!)

See here for the only picture I can find
of what it used to look like....