Couldn't you get some black highlight makeup? Over here in NZ we have this stuff which you can get worldwide called... Champion Tails Highlight Makeup. It comes in several shades, including black and here's what it says:
Designed to darken around the eyes, muzzle and dark points of the legs, for that winning show look. Available in 75g soft squeeze tube with applicator nozzle, and 250g pot.
We use it on all our show horses, whatever colour we desire. I think it'd be much easier to apply, and it is made specifically for horses. We just use a sponge, and apply it that way...
Here's my pansy man with his makeup on (he thoroughly loves it over his eyes)... I know he's dark bay, but we use it to take his "mousy" look away, and even it up with the rest of the colour of his coat. WITHOUT MAKEUP:
I didn't either, I laughed when my best friend told me she put makeup on horses... she did the above handiwork with my horse, Evo. I've seen it really enhance a horse who, for no better words, needs to stand out a bit more. Of course, if you put too much or too little on, just like with humans it doesn't look right. Showing is hardcore... I've seen so much that I never thought existed as a show groom haha!!! x
I don't show halter but I have a baldie. I used maybaline brand, it worked fine. It wore completly off by day two. I ended up going the tattoo route. It's been almost two weeks, and the tattoo has faded alot. To a grey like color, but honestly it still looks pretty natural.
Here's her befor pic. I don't have any recent one yet.
My second horse was an Appy who had that same sunburn issue around the eyes and nose. I used the same sunblock as the lifeguards use..the white cream; Zinc Oxide. Being a lifeguard myself at the time I had plenty of it around :)