A new cultivar of Hydrangea macrophylla named `Blue Knight` that is characterized by its early blooming habit, its ability to re-bloom on new growth after deadheading, its consistently well-formed, lacecap type inflorescences with large, double, rounded shaped sterile flowers that are light to dark blue under acidic conditions and bright pink under alkaline conditions, its deep green glossy foliage, and its strong stems that enable pot culture without staking.