Certified GKH Gardening Expert
Blossom end rot is a common issue when growing zucchini (squash), as well as tomatoes, and yes, this is due to a calcium deficiency. The best way to add calcium to your garden is with compost, as most compost is fairly rich in calcium (which the addition of lime or eggshells to your pile can ensure). You also don't need to worry about over applying calcium, which is another common issue. Another approach to adding calcium is to find an organic calcium foliar spray that you can spray directly on the plants (follow the directions). Generally, you only need to add calcium on an as needed basis.
These articles should help: https://www.gardeningknowhow.com/edible/vegetables/squash/squash-blossom-end-rot-causes-and-treatment.htm