I have done research on blood and in particular the component adding color and transferring oxygen.
Hemocyanin is directly dissolved into the blood. This works for an octopus, but not a reptilian alien. Hemoglobin however is contained inside RBCs and very little is directly dissolved into the blood. This could work for a reptilian alien but it seems too earthlike to me.
So I figured that maybe I could have the Hemocyanin compound inside globin proteins inside cells and these cells would be BBCs or blue blood cells.
Each Hemocyanin has 2 copper ions that can bond with 1 oxygen per ion so per molecule that is 2 oxygen molecules transported, double the capacity of hemoglobin. Other than that it is similar to hemoglobin(each protein has 4 subunits and each subunit has 1 Hemocyanin molecule.
Now, arterial blood with Hemocyanin looks blue because it is blue. Venous blood with Hemocyanin however is colorless. So the only way you could see veins is by seeing the tissue itself and that is pretty much impossible without an incision. So the only blood vessels you would ever see with the naked eye are arteries.
Is cyanoglobin a plausible way to have oxygen transported in a reptilian alien who most likely has a totally different evolutionary tree from ours?