Same here. My plant is 2 years old. This year it is much taller but thinner. The green is going from a green into a yellowish green now. Do I need to add fertilizer or some mulch around the base now? We are now in 90 degree weather. It gets plenty of water once a week, or sometimes twice. This has never been a problem before.

I fertilize mine. The yellowing could be a lack of nitrogen.

Would a lack of nitrogen show up so quickly?

Are these yellowing Black and Blues in full hot sun?

SG

Yes and no to the full hot sun. When I first planted it in early May we had a long string of cool rainy weather. Now it's hot and dry. It seems like the poor thing started turing yellow way before the hot spell.

The leaves of this salvia will turn yellow out here when it is on full or mostly full hot sun.

Mine all get a good amount of shade during the day.

Try keeping it watered as you just planted it this spring.

Most salvias are drought tolerant after their first season in the ground.

Some salvias are water hogs. I think S. guaranitica 'Black and Blue' falls between being a water hog and a drought tolerant salvia.

SG