Boxlid
Be careful to not get alky or solvents on the sides if you give the blocks a quick wipe, might take off the black color. Great temps on 1.5mm padding too if you prefer more cushion for the... well never mind 
The color will not come off. Delrin is the brand name for a plastic called acetal. Nate isn't painting the acetal, so therefore it is solid black throughout, as the color is integrated to the plastic when compounded.
Here's a chart of common plastics and their chemical resistance levels. Acetal is the third column from the left. Alcohol won't have any affect on acetal (Delrin)
http://www.plasticsintl.c..._resistence_chart.html The reason you see Delrin being so commonly used for something like a waterblock or other short run production, is that it is easy to machine. It's not too brittle and it doesn't melt as badly as other plastics from the friction of the cutting tool. Typically, plastic would be injection molded and not machined, but for small volume production like waterblocks, it's not feasible due to the cost of the tooling (the mold) which can cost $10,000 and up for something that size easily.
This is probably waaaay more info that you wanted, but I work in the plastics industry so I figured I'd explain for those who are interested.