There are 3 possibilities to link Hosur line to YNK line.
1. Near Lottegollahalli, by way of a bridge across outer ring road. requires more engineering, but definitely possible.
2. Near SMVB terminus. Trains can make a circular route and move to other line. This is what you mentioned as black line. Definitely possible, but will should be there.
3. Near SMVB bypass...
more... line, over Kasthuri Nagar from Hosur line to YNK line. Impossible as thickly populated areas. Also this will bypass SMVB. Hence not possible.
My friend suggested a down ramp from Hosur line to KJM station, to which I replied saying that it is not possible and also it won't help traffic to YNK line without loco reversal. Even now, with a loco reversal, at BYPL trains can be operated from Hosur to YNK side and KJM/ WFD side.