Abstract: As the sole light source in a plant factory is light, optimization and regulation of the light environment are the most crucial ways to improve the yield and quality of fruits and vegetables. Blue light plays an important role in regulating plant morphology and photosynthetic characteristics. Previous studies on continuous light have mainly focused on red and blue light, and there have been few studies on the utilization of blue light to optimize plant morphogenesis. To explore the effects of different intensities of blue light on the growth and morphological establishment of fruits and vegetables in plant factories with continuous light, this experiment took “Tiberius” lettuce as the research materials and divided the 24-h cycle into two periods. Period one (H₁) was 18:00–10:00 the next day, while period two (H₂) was 10:00–18:00. During period H₁, the light environment conditions of each treatment were the same, the light intensity was 200 μmol·m⁻²·s⁻¹, and the red to blue ratio was 4∶1 including the control group (B0, without light) and three light intensities of 28 μmol·m⁻²·s⁻¹ (B28), 60 μmol·m⁻²·s⁻¹ (B60), and 100 μmol·m⁻²·s⁻¹ (B100). The results showed that 1) compared with B0, the plant height and the angle between the stem and leaf and the horizontal plane of lettuce under treatments B28, B60, and B100 were significantly increased. With the decrease in blue light intensity, the plant height of lettuce increased, the angle between stem and horizontal plane increased, and the leaves were slender; 2) compared with the treatment B0, the stomata density in the leaf abdomen under treatments B28, B60, and B100 decreased by 30.8%, 34.6%, and 25.0%, respectively, and the leaf thickness decreased by 15.9%, 28.4%, and 8.8%, respectively; 3) blue light promoted the increase in chlorophyll a, chlorophyll b, and the redistribution of the chlorophyll a/b ratio, among which the highest chlorophyll content was observed under treatment B60, whereas no significant difference in chlorophyll content was found among treatment B28 and B100; 4) compared with the treatment B0, the transpiration rate under treatments B28, B60, and B100 increased by 30.8%, 38.5%, and 30.8%, respectively, contributing to the absorption of nutrients by plants. The highest nitrogen, magnesium, potassium, and iron ion contents were obtained under treatment B28, which increased by 18.2%, 33.3%, 65.9%, and 72.7%, respectively, compared to those under treatment B0. In summary, blue light supplementation during period H₂ was found to enhance the height of lettuce plants, widen the angle between the stems and the horizontal plane, and promote a more compact shape. Interestingly, lower blue light intensities resulted in greater increases and more pronounced morphological changes. Additionally, blue light supplementation during period H₂ led to a reduction in leaf thickness and facilitated the accumulation of photosynthetic pigments, but increased stomatal conductance, which is beneficial for the absorption of nutrients in the leaves. Therefore, in a plant factory, continuous light combined with different blue light intensities can purposefully regulate the morphology and photosynthetic characteristics of lettuce, thereby increasing the number of cultivated plants and the yield per unit area.