Try (RGB) = (73, 152, 255) with 35% opacity. Numbers have base=10. No guarantee, I cannot be sure how close the shown image is to your original. Can easily cause 1 unit errors due roundings.
ADD: It's calculated with the well known elementary mix formula. RGB component A in top layer with opacity P and the same color component B as opaque in bottom layer give mix PA+(1-P)B. assuming everything happens in the same color space. That's a strong assumption, I guessed your image was originally sRGB and your system obeys the same mixing as Photoshop by default with no mixing gamma settings.
The elementary (some of us would say naive) mixing formula gives enough equations from where the opacity and R,G and B are solved. Actually there are 2 more equations than those 4 which is the minimum. They could be used for finding the best match. Imgur image service and web applications between us change your image and as said, the mixing process in your system was only guessed to be the same as in Photoshop by default.