Urban land use land cover (LULC) change raises ambient temperature and modifies atmospheric moisture, which increases heat-related health risks in cities. Greenspace and bluespace commonly coexist in urban landscapes and are nature-based heat mitigation strategies. Yet, their interactive effects on urban thermal environments are rarely assessed and it remains unclear how extreme heat events (EHEs) affect their ability to regulate human thermal comfort. Using multi-year observations from a dense urban observational network in Madison, WI, we found that green and blue spaces jointly modify the intraurban spatiotemporal variability of temperature and humidity, and the resultant effects on thermal comfort show diurnal and seasonal asymmetry. Greenspace is more effective at cooling throughout the year, particularly at night. Accelerated cooling efficiency is found in areas with dominant greenspace coverage and little co-influence from bluespace. The thermal comfort benefit due to greenspaces can be offset by bluespaces because of intensified nighttime warming and humidifying effects during the warm months, although a weak daytime cooling of bluespace is observed. EHEs enhance bluespace cooling, but the overall joint thermal regulation remains the same due to the enhanced moisture effect. Our findings suggest that diverse outcomes of green and blue spaces cross multiple temporal scales should be holistically assessed in urban planning. The analysis framework based on generalized additive models is robust and transferable to other cities and applications to disentangle the nonlinear co-influences of different drivers of urban environmental phenomena.