You're unlikely to find a single, definitive "EPA rating" like you would for a modern vehicle. Instead, you might find estimates or anecdotal accounts from various sources. These sources would be unreliable due to variations in driving style, vehicle condition, and the specific engine installed.
To get even a rough idea, you would need to specify the exact engine size (e.g., 6.2L) of the 1985 Chevrolet Blazer diesel. Even with that information, finding a reliable EPA equivalent would be difficult.