Darkening leather is really not a criminal offence, but it should be a consideration depending on how you want your leather item to turn out.
There are many ways to "skin a cat" when it comes to leather treatment, and I can't say that any out there really do damage.
Your frog I would classify as a heavy leather, vs a light leather that might be in a bomber jacket.
Heavy leathers, be it harnesses and saddles ect. need deep treatments to do more than just treat the surface.
On new reproduction leather, be it a frog or a sling to go on your 03-A3 I actually soak then in Neetsfoot oil overnight, let them hang to dry and then wipe off the excess for quite a period of time.
Sometimes darkening might be the desired effect, and any treatment should be tested before a full blown treatment is tried.
The 2 repro frogs shown were the same light color, but with differant treatments came out differant shades. (now if I could only figure out how to antique the shiney buckles) LOL
According to Rawhide Johnson who works with the Buffalo Bill Museum in Cody, WYO, he said to use several treatments of a good grade of olive oil and then after a day or so of drying, top off with a light coat of Saddle Soap "glyserene" to add the shene so that dust does not adhere.
Neetsfoot oil is okay but is said to rot stitches, although Rawhide has said he has never seen it happen. Neetsfoot oil also remains oily and attracts dust.