Slade initially clocked a 1:20.6486s just after two-and-a-half hours.
After six hours, he dropped in a 1:20.5096s, and he ended the day 10th all told, 1.0007s down on Brown's morning flyer.
The 2021 season marks the first time Blanchard's car, backed by family business CoolDrive Auto Parts, will be operated out of a fully-fledged Supercars team.
Blanchard, a former full-time driver himself, will run the Melbourne-based operation out of CoolDrive's head office.
He has already landed several coups, with Slade to be engineered by former Erebus guru Mirko De Rosa, with Brendan Hogan shifting from Tickford Racing to manage the team.
The Winton test also gave Hogan and De Rosa a chance to work alongside Slade in official conditions.
The new season will be Slade's first full-time campaign since 2019, having been left out of a drive for last season following his exit from Brad Jones Racing.
The 2021 Repco Supercars Championship will commence next week at the Repco Mt Panorama 500. Tickets are on sale now.