My advice would be to have a professional do it the first time. At least then they can give you tips for aftercare and what not. Plus you'll have to get the color on your hair evenly to avoid blotchiness. If you have long hair that can be hard to do.

You can also use Henna, It's shorter lasting, but if you leave it for a long time it will cover well.

Well, you know the moisturizing drill by now {having bleached hair}. How about protein filler? Whenever I go darker, ESPECIALLY with semi or permanent dyes, I'll do one every week for a month until dying it dark. Fills in the hair shaft, so that the dye will 'take' more evenly.

I settled on knox gelatin {no sugars or anything, just straight~up gelatin}. the drawbacks of protein fillers is that your hair may seem to have more britte ends if not tended to {diluting shampoo, rinsing hair with h20 daily, hydrating conditioner}, but again, after just going with simple gelatin that problem stopped for me. Also, if doing them without dying hair, they tend to yellow bleached hair. not much, but if one wishes to stay platinum, its best to use a toner with these.

I've done this many times. This is what I've learned if you don't want your blond hair to go ugli-ish gray you need to dye it a middle color first. The in between color should be a reddish tone. You can try browns, but red is more interesting and if your are flexible with how bright the shade comes out- even better. You MUST use a semi permanent dye for the in between stage. It is likely that your hair is too porous to hold a permanent- it will wash right through. THEN wait like 2 weeks and dye your hair with a semi -permanent black. It will look good right away, but wont stay long, it will rinse out in a few weeks but will STILL further darken your auburny (now) base. Your hair will be ready about this time for a permanent black dye,
Good luck- I hope this helps<3

Judging from your recent pics, you were successful! :)