I would think that the e-coli was able to take hold of the bird because it was already weakened from the blackhead as i have read that e-coli is prevelent with birds around so the bird was more sucetible to getting it. the bad thing is peas hide their illness well and by time it shows any symptoms it is already well under the way to being very sick before you know and treatment is often times to late.
Vets that know how to treat blackhead will often prescribe metronidazole, Fenbendazole and a 2nd antibiotic to treat any of secondary infections.