Blood in a just fresh udder is usually the small blood vessels bursting from the pressure of an increased lactation over last year. Especially when we hand milk every 12 hours, in a doe nursing kids the udder gets empty all during the day...sometimes does like this when you wean her kid, expand the udder for the first time and then you get blood. You can also milk pink milk from an injury, long pendulous udders are always in harms way of being stepped on, and fighting does of course seem to aim for each others udders!
After you have blood line this it's nothing to see some real tissue, since the cells slough off. It's not harming the udder, it's actually a good thing, more milk!
It can take a few day to a few weeks for them to get over it, and some don't until 6 to 8 weeks comes and they naturally even out their flush of milk.
Mastitis is heat, nasty milk you would not drink, a hot udder and a very sick doe...subclinical mastitis is poor keeping quality to the milk, a thickened feeling to the outside of the skin of the udder (not fat inside) and udders that are lopsided, not from kids nursing..that's staph and its everywhere!
If you have to ever say to yourself "Hmmm, wonder if she has mastitis" than she doesn't because when you get a doe who has mastitis believe me you will KNOW! It's like being a little pregnant
You didn't do any harm, and I have no idea why you are seeing an oily residue on her milk, I know at the end of lactation my Nubians have very high butterfat after milking for 10 months, even my cheese has an oily slick to it, a yummy oiliness, but it is there. Is your grain high in fat, feeding black oil sunflowerseeds also? What breed? Vicki