Peat moss would be good to top dress but not for turning your hydrangea blooms blue (at least not quickly). It is the aluminum in the soil that turns big leaf hydrangea blooms blue (if they are predisposed for changing colors). Acidic soil allows the plant to absorb the aluminum while alkaline soil blocks the absorption from all I've read. To turn the soil acidic you can apply ammonium sulfate or sulfur-coated urea.

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Raise acid level

Peat moss can be used to acidify soil (which is what helps keep the blue in hydrangea flowers) but it is best used as a soil amendment at planting time rather than a mulch later. Peat moss used as a mulch dries out too easily and then becomes hydrophobic or repels water.

Aluminum sulfate, aka hydrangea bluing, is a better choice after planting. This will help to acidify soil as well as supply aluminum, which is what actually causes the blue color development. Just apply according to package directions. But be aware that this is not a fast fix (nor is the peat) - it is best applied in fall or in early spring to get good summer coloration. IOW, likely will not make a difference this season if applied now.

It's also important to consider your natural soil pH. It is extremely difficult to make substantial changes in soil pH nor will they be very long lasting. I know a great deal of MO has natural limestone outcroppings which tends to make soil more on the neutral to alkaline side. If this is the case in your area, it may be very difficult for you to acidify your soil sufficiently for the hydrangea to bloom blue. You may be better off growing this plant in a container or raised bed where you can tailor the soil specifically to the plant needs.

Trying to make a hydrangea bloom blue in an alkaline situation is an exercise in futility. Enjoy the pink.

Linda

I have had great luck with aluminum sulfate. I have 20 or more similar to Endless Summer that I maintain as blue. My soil is very alkaline and they were bright pink for the first two years.

Use the aluminum sulfate sparingly. Error on the side of too little and add more next year if needed. It took me two years to get them where I wanted, but it was worth it. The maintenance is actually very low.