For other walkthroughs, click PLAGUE INC MEGA BRUTAL WALKTHROUGHS tag at bottom of this post.
Android only, on my tablet - I don't know if it works on Xbox One or any other device.
I found the difficulty with Black Death was getting everyone to infect quick enough, without killing them all off too early - if the game mutates Total Organ Failure on its own, you might as well give up and start again!
I've played it a few times using this exact sequence, the 2nd time it didn't work because New Zealand and the Phillipines didn't infect.
DNA Genes: Cytochrome Surge
Travel Genes: Teracyte
Evolution Genes: Patho-Stasis
Mutation Genes: Genetic Mimic
Environment Genes: Xerophile
Start the plague in Egypt. India has worked for me in the past, I've tried Brazil and that's hopeless.
Activate in this order:
Drug Resistance (DR) 1
(Note nice balance of infectivity and protection!)
About now, they'll be starting the cure.
Genetic Hardening (GH) 1 and 2
Cold Resistance (CR) 1 and 2
Sweating (it's hard to make it spread in Russia, Canada and Greenland; these help)
I had loads of points around now but the cure was still pretty low, so I concentrated on getting everyone infected, you see!)
Blood 1 and 2
Extreme Bioaerosol (EB)
During this time it mutated Paralysis, and Cysts; I devolved cysts to get 2 points back, but kept paralysis.
At this point the whole world was infected, with the cure at 60%.
I devolved EB, Air 2, CR1 and CR2 to get 6 points back.
Genetic Reshuffle (GRes) 1
GRes 2 (when cure at 67%)
During this time it mutated Immune Suppression, Sneezing and Hypotension; I kept them as I wasn't desperate for points and I thought it probably wouldn't refund me any, anyway.
When the cure was at 81% I got Hardened Reshuffle.
By then the cure had slowed right down, and I spent the remaining points on a couple of things like Systemic Infection, to speed up the end of the world!
Ended with the cure at 94%, game finished giving me 23806 points.