Replicating a layout I saw on the NHS Home page. Negative margins are used to drag the blue box down. I created an SVG upside down diamond of the correct colour in Logoist5 and then set negative margin on the Y axis and left margin to position the diamond in the desired spot.
Possibly would need to have a different layout on mobile.
Image from Shutterstock
Fine tuning the positioning of the text box very much depends on what photo is being used.