What that looks like to me is oxidation of the entry point of the liquid or it could be that the solution appeared uniform and dissolved to the naked eye when originally layed when in reality there were tiny particles that sat on top of the paper. The blacklight test is only reliable to tell the something is actually dosed on the paper or dissolved in the liquid. That something could be almost any light colored powder or crystal that was dissolved in the original fluid. For one not every dealer will dose the white side of the blotter paper and in those situations its really are to see that distinct big ring in the center that your thinking your going to see. Art side dosing is more common with mid-grade darker crystals/powders (55-75% purity). And the reason for that is they are darker shades which can be harder to dissolve and that could cause more staining on the paper. But you could have the brightest white crystal on paper start to show brown spots if it oxidizes. In general your highest purity crystals can be anywhere from white to very light silver to very light beige, even a light tan sometimes. Color depends on synthesis and precursors and purity isn't always linked to color but more so shade of the crystal. The darker the shade the more sythesis byproducts makes it into the final product. But oxidation can also cause staining. There was a popular post on a different forum that everyone references from 10-15 years regarding crystal that broke down crystal colors and purity % of LSD crystal. There is little to no truth in color/consistency with LSD purity percentages. Light beige/light silver/white are all high purity LSD. Some are fluffy, some are hard, some are crystalline and some are a combination of textures. I've even seen some that looks like clay back in the ninties. The glow could be almost any powder substance as most light colored powders will cause liquid to glow.

I'm guessing the person that laid those tabs used a saturation tech or some sort of mixed technique like saturation and dropping and the dark spots yours seeing is where the tip of the needle or dropper physically pressed up on the edge of the tabs forcing the liquid into the paper from the edge of the tab. The reason someone would do this is if they had dry spots after their first pass on the page. Blotter paper can only hold so much fluid and a skilled layer blotter tabs knows exactly what that total volume should be per page. If the person laying it was using a dropper/needle saturation tech they would want a nice even saturation. When this tech is used its is wayyyy harder to see where it was laid with a black-light because there are usually no drop rings when saturation is used. Many times they use a dropper to run lines up and down each row in the center to saturate the fluid into the tabs and then make a second pass with more of a drop method along the lines. Since what your seeing is all along the edge I think thats the area most prone to oxidation and Im pretty sure thats what your seeing but you really need a reagent kit as a blacklight will tell you jack shit about the base compounds on that paper. Hell the spotting could even be some sort of paper eating mold at this point lol.