Blood is frequently encountered as physical evidence in cases of violent crimes. Detecting the presence of this crucial bodily fluid is a fundamental task in forensic investigations, and various presumptive tests are employed for this purpose, with the forensic luminol test being one of them. Traditional catalytic tests like phenolphthalein are commonly utilized to quickly screen suspected stains for the presence of blood. These colorimetric tests are not only cost-effective but also provide rapid results, making them suitable for on-site screening of potential bloodstains directly at the crime scene. Here, we investigate KM reagent and Indigo Carmine reagent for latent blood trace detection by colorimetric analysis. The OpenCV for assisting in the interpretation of reagent color changes was employed. The results show that the sensitivity of KM reagent is better than Indigo Carmine reagent, and the limit concentrations they can detect are 0.01 % (w/w) and 0.1 % (w/w) respectively. However, both reagents have a false-positive by bleach containing sodium hypochlorite.