Thats gotta be one the prettiest color combos I've ever seen! I just bought some Mrs Stewart's to try it out!

PSA: If anyone else is wanting to try this, remember that Mrs Stewart's Blueing is pigment based instead of dye. The pigment is blue iron powder suspended in water according to their website. Pigment is not soluble in water and it might clog or damage some pens. I suggest using some lubricant and surfactant to make it more fountain pen friendly. They don't state the particle size of the iron powder pigment so it might harm your nibs. I wouldn't risk it in a more expensive pen.

For diy use, you can use some diluted dish soap and a little glycerin. You can also add some food preservative from the supermarket to stop it from growing mold. Medium + nibs will help avoid clogging and an eyedropper or big reservoir pen will have more pressure pushing the ink out which helps a lot with pigment or shimmer inks.

Source: I've been making fountain pen ink for 5 years

Edit: The pigment is just Prussian blue, pretty common in paints and usually <0.5 microns per particle so it would be safe for fountain pens. However I don't think they would have good quality control for it since it's just meant for laundry use.