This signal box was in Renfrew, controlling access to a looped headshunt for a short line north, on a tight curve, to Lobnitz's Engineering and Shipbuilding Works and Simons' London Works. The headshunt was approached from the Glasgow direction (the east) and on the north side of the line. The signal box was located in the 'V' between the main line and the branch curving north. The box was named for Blythswood House and estate which were to the west.
The connection and box closed in 1932 (after the Grouping) as it duplicated an older connection from the Paisley and Renfrew Railway from Meadowside Junction. (Both connections were owned by the LMS after 1923, with Blythswood box only used as required from 1926.) The reversing spur and its loop were removed in addition to the curving line north.