Actually, I'd ask the opposite question: what percentage of electrical or software engineers are black?
You can't live in San Jose unless you have an upper-middle class income --- the housing costs are too high. And, the only people that are going to pay those costs are the people that want to live close to work. Since the upper-middle income jobs in San Jose/Silicon Valley are largely technological, that's who you should expect to live there.
If you accept my premise, the question should be: why are blacks under-represented in the technology professions?
That was my point also.