Blue water, defined as the surface and sub-surface freshwater available for human use, is increasingly under stress due to rising demands and altered precipitation regimes driven by climate change in semi-arid regions. Despite this, comprehensive assessments quantifying blue water scarcity and its response to historical and future precipitation variability remain limited at the river basin scale. This study quantifies the blue water scarcity index (WSIbw) for the Mahi River Basin (MRB), India, by integrating blue water availability with sectoral withdrawals from the domestic, industrial and agricultural sectors. A sector-wise analysis of water withdrawals was conducted to estimate the contribution of sectoral water withdrawals to WSIbw. Subsequently, we investigate the sensitivity of WSIbw to precipitation variability over historical (1981-2015), near-future (2031-2065), and far-future (2066-2100) periods using precipitation anomaly-based classifications for Shared Socioeconomic Pathways (SSP) scenarios: SSP1-2.6, SSP3-7.0, and SSP5-8.5. The major findings are (1) agriculture dominates blue water demand in the MRB, contributing the largest share to WSIbw; (2) precipitation variability strongly influences WSIbw, with dry years exhibiting deviations up to 30 % from the multi-year average and even moderate dry/wet years showing significant impacts; and (3) under future climate scenarios, the relative change in WSIbw is projected to intensify markedly, particularly in the upper sub-basin districts, with increases exceeding 60 %, exacerbated by concentrated anthropogenic activities. This study provides a robust framework for assessing blue water scarcity under dynamic climatic conditions and offers actionable insights for regional water resource planning and adaptation strategies.