198ci
Yup, the only colour Slant-6 engines were painted in the 198 years (1970-'74) in the North American market was blue.
Slant-6 colours from the factory:
Metallic deep turquoise: Some 1960-'61 Plymouth-Dodge full size cars, North American market
Silver: 1961 (RV1 sold as "1962") Valiant, Australian market, some '60-'61 Plymouth-Dodge full size cars, some '61 Valiant/Lancer.
Red: 1962-68 all passenger cars, all markets, except as specified below, plus most 1960-61 Valiant/Lancer, North American market
Turquoise-blue: 1962 thru 1966 Canada-market 170 engines, plus all slant-6s in 1969 passenger cars & trucks throughout the North American market
Blue: 1970-1982 all passenger cars & trucks, North American market
Turquoise-green: 1967-1969, w/2bbl carb, Australian market
Orange: 1967-19??, w/2bbl carb, South African market
Yellow: 1962-early 1969 truck/bus, North American market (some '61-'62 got red)
Black: 1983-up, all applications, North American and probably other markets.
This list almost certainly omits some versions of the slant-6 sold in some non-North-American markets.