Sod several months old, now has several areas of "rusty blades". See picture.

[MowerMark]

Our old bluegrass lawn was >20 years old and we never had the problem described below.

A few months ago 80/20 Blue/Rye sod was installed. Watering 20 minutes 3x/week starting at 5AM. Mowing to 1.5 inches. In the past two weeks, areas of "rust" has appeared on some blades, but not in all areas of the lawn.
We have used professional high nitrogen fertilizer every month, more than recommended.

Article on grass rust (and it doesn't seem like rust is the problem): https://www.thisoldhouse.com/lawns/what-is-grass-rust-and-how-to-remove-it

Anyone here know what this is and how to cure it?

 

[Auto Doc's]

Our old bluegrass lawn was >20 years old and we never had the problem described below.

A few months ago 80/20 Blue/Rye sod was installed. Watering 20 minutes 3x/week starting at 5AM. Mowing to 1.5 inches. In the past two weeks, areas of "rust" has appeared on some blades, but not in all areas of the lawn.
We have used professional high nitrogen fertilizer every month, more than recommended.

Article on grass rust (and it doesn't seem like rust is the problem): https://www.thisoldhouse.com/lawns/what-is-grass-rust-and-how-to-remove-it

Anyone here know what this is and how to cure it?

Cut back on the water and definitely the fertilizer. Let nature take care of itself without so much intervention.