To change hydrangea blossoms from blue to pink, you need to increase the pH (acidity) level of the soil that it's planted in. This can be done by applying lime to the soil. Additionally, to change the blossoms from pink to blue, you should add pelletized sulfur to the soil.
You should only place mulch in your garden once a year, but it can be done in any season. The most popular time to add mulch is the Spring because that's when most gardeners are cleaning up and adding/removing plants.
There are several factors to take into consideration when planning a watering schedule for your plants; like how long the plant has been growing where it's planted, and how much rainfall and or sun it gets daily. You should always check the soil for moisture before watering a plant. If there's moisture, the plant probably doesn't need watered; if the soil is dry, it needs water. If it takes 3 days for the moisture to dissipate, then you can know that the plant probably needs to be watered every 3 days, but should still be checked each time.