i do believe that gaytri is talking about her bad blood that is flowing in akaash veins...by the way i do believe that the blood that is considered is the blood of the father and since yash is sp son he has sp blood...but we now see where the BAD BLOOD is coming from...and it belongs to her and akaash...so she is really speaking about herself and her only son akaash...
I feel that blood comes from both father n mother so all 4 sons have the same blood of SP. Even if Yash has half blood of Radha it doesn't mean that his (whole) blood is ganda (as per the theory of G3 not mine).
SP was a equal party for the mistake or rather has more share as Radha was not aware of his marital status. So his blood is ganda and if so, blood of rest 3 sons is also ganda.