you have got to be kidding me...
do not strike any thing with you finger tips.... no thrusting fingers in sand or any other material( there are exceptions).....the meridians in your fingers and hands connect to the internal organs and this repeated shock can damage them... and yes..it can lead to blindness and other side effects...there is a better method ...however I do not know anything about you to know whether you are ready for this type of training..but if you are asking on this forum you might not be...if you are really interested to learn the proper way let me know.....