You can check these papers

1. Carbon doped PDMS: conductance stability over time and implications for additive manufacturing of stretchable electronics

2. Polydimethylsiloxane-based conducting composites and their applications in microfluidic chip fabrication

How are you dispersing the carbon black in your PDMS? Even if it 'looks' black, it may be that there is still substantial agglomeration of the carbon black in your material. Smaller aggregates lead to increased conductivity, see for example

https://link.springer.com/article/10.1007/s00396-002-0718-8

Sooner this negative effect is a result of poor dispersion of CB in PMDS matrix. When we prepared silicone composite filled with carbon black, my PhD student introduced CB into silicone resin very thoroughly. She mixed silicone with CB in a mortar and added CB by portions triturating each portion approximately 5 minutes. Of course, better to use some convenient equipment (for example, two-screw miniextruder or brabender) but if you don't have it, the manual method is quite fit if you are patient enough. Your 15 % of CB by weight means about 7 vol.%, we had already conductivity at this CB concentration. I should say that we also studied piezoresistive effect and found that silicone/CB composites gave not so good results in cyclic testing (we tested the composites by compression mode). Electrical signal wasn't stabile with increasing number of cycles loading/unloading. Unfortunately I can't send you our publication, it published in our national scientific journal in national language.

Thank you so much Prof.Mamumya for kind help. I did tried mortar and pestle and it worked. I got 23 kohms sheet resistence. However, regarding poor piezoresistivity, can you suggest any other filler which according to you gives better response. 

You can use a  polymer like polyaniline or PVDF to enhance conductivity and piezoelectric effect.