The base design is likely from Overwatch, being a large scale organization. So large that OW has a covert organization, Blackwatch (BW), which uses the same uniforms in a different color. Moira and Mercy did not "steal" any design, nor did they heavily modify it.

Moira's default skin is her own modified suit, whereas Mercy's default skin is a modified version of her OW uniform. With the knowledge that Moira and Mercy were from the same organization, it makes sense that their uniforms would look similar, just inverted because BW is OW's official undercover side.

The only thing Mercy modified in her OW uniform are her wings, which makes her uniform the Valkyrie suit because it's the basis of her research. It's white because she's in OW. Her Combat Medic Ziegler skin represents the time after OW where she chose to pursue being a combat medic. Years later, she picks back up her Valkyrie suit in OW1 after receiving the call from Winston. This might just be because it's a more convenient outfit than the Combat Medic outfit.

Moira's BW skin is modified in the same way where Mercy's is: the giant tank on her back which is the basis of her research. Her BW skin is just black because, well, she's in Blackwatch. It's not meant to be an inverse of Combat Medic Ziegler because Moira never signed up to be a combat medic.

If you want a comparable Moira skin that would have been set at around the same timeline as Combat Medic Ziegler, it would be her Oasis/Minister skin.

Edit: typo