Uses of Russian engines cannot be a good reason not to sell it to BAF. China is trying frantically to develop its own engines, but still the Chinese engines remain immature. So, China uses Russian and Ukrainian engines. THese countries' jet engines are fully developed because of a long history of their development process. As far as I know, even FC-1/JF-17 uses Russian engine.
The "immaturity" tag of Chinese engines is a bit out of date in my opinion.
The WS-10A has been powering J-11B twin-engine fighters since 2010.
The WS-10B equipped J-10B probably would take no longer than a year or two to be ready for service.
Even if BD places an order for J-10 today, it would not be delivered till around 2015 anyway.