The odds of them losing militarily are virtually nil. They could face an insurgency, but there isn't a whole lot of rural Taiwan for insurgents to vanish into and occupying cities is a lot easier absent language and cultural barriers. The could be isolated politically and economically, but realistically China's territorial claim on Taiwan is on far firmer legal and historical ground than many other territorial disputes (eg their control over Tibet).
I don't see the US involving itself directly. What are they going to do, counter-blockade? Start a naval shooting war with a full-on nuclear power on the other side of the world? I don't see Japan backing that either, despite their natural anxiety over the vulnerability of the Ryukyu islands. Support for US bases in Okinawa is ambivalent at best, and while Japan is surely not thrilled about Chinese regional hegemony it's also a reality they've dealt with for thousands of years.