OK the DX840 is PPG's blending solvent and it's a very good idea to use the blending solvent from the same manufacturer that you get your paint from. Blending solvent is basically a very slow reducer (it does have other properties in it that give the paint more teeth to bite into the existing finish).
If your going to use PPG, the Acrylic Urethane is called Concept (I'm from Canada and I don't know if in the US they called the Acrylic Urethane Deltron Concept or just Concept). As I'm sure you are aware, you will need to first repair the scratch (if you need information on this let me know and I will try and walk you through the steps involved). After you have filled the scratch the trick is to keep your repair area small without any hard masked primer lines. The best way to achieve this is to take a piece of masking paper with masking tape on it and basically lay the masking paper down allowing the paper to cover up the repair and then lift it up so that the repair is exposed (about 2 inches on all sides of the repaired scratch). By reverse masking you will create a soft edge and when you apply 2 coats of primer, there won't be a solid line of primer that you need to sand off. After it's been primed and the primer has cured use a small block and prep the area that has been primed with 400 grit dry or 600 grit wet paper...I prefer 600 grit wet, it's seems to leave a better finish.
Now you have your repair complete reverse mask the graphics about 1 inch away from the graphics (the same way as you did for primer) this, if your scratch was 15 inches away from the graphics, less the 1 inch you reverse masked at the graphics and the 2 inches you applied primer on the scratch, should give you 12 inches to do your blend. Prep the area around the scratch with a white 3M Scotch Brite pad all the way up to the masking paper protecting your graphics and 12 inches below and 12 inches on both sides of the repaired scratch. Mix up a small amount of paint with the proper hardner and reducers outlined in the tech sheets. Now you will need to set up your paint gun to paint a small area, turn your fan in so that you have about a 2 to 2 1/2 inch pattern, also turn your volume of paint down and adjust your air pressure so you are spraying at the recommended pressure. If you have 2 paint guns, that would be a benefit now, one for paint one for blending solvent...if you don't...apply the paint until you have coverage and allow it to flash between coats, immediately after you have achieved hiding, remove about 50% of the paint in your gun and replace it with DX840 blending solvent, stir the paint with the blending solvent thoroughly put it back in the paint gun and purge the gun (you will still have 100% catalyzed paint in the chamber of the gun which you can remove by triggering the gun or purging...just like nitrous). Now open your pattern up so that you have a 3 to 3 2/3 inch fan, check and adjust your air pressure, your volume should remain about the same because with the blending solvent your thinning or lowering the viscosity of the paint and apply this solution around the area you have just painted...again, remove 50% of the paint and blending solution in your gun, replace what you have removed with blending solvent and apply another coat extending each coat past the last and repeat until the entire prepped area has been painted, do not apply paint past the prepped area...it will have a tendency to peel. If you find that any area of the blend is getting dry, apply a coat over that or the entire area to keep it wet and maintain gloss. When the entire prepped area is covered you are done, remove the masking paper and allow it to cure, once it has cured thoroughly you may compound and buff (I prefer to hand polish...less chance of the fresh paint peeling back). If you did everything correctly, you should have an invisible repair.
Now, I do need to mention, doing a blend in clear coat is difficult, doing a blend in Acrylic Enamel can also be very challenging...doing a blend in Acrylic Urethane is even more difficult than either of the other 2 products, it takes a lot of practice and knowing exactly when and how to apply the products and many professionals have trouble blending, let alone a metallic Acrylic Urethane.
I hope this helps, if you need more information, let me know and I'll try and be more clear, or, if you have access to two paint guns, I can outline the two gun method for you (I personally have had more success with the two gun method when blending Acrylic Urethane than with the one gun method).