I find this hard to believe. There are 80 cases of 1d20 + 1d4, your answer must be a multiple of 1.25%.
Assuming you need 11+ to hit, bless will help in 10 cases:
- 10+1, 10+2, 10+3, 10+4
- 9+2, 9+3, 9+4
- 8+3, 8+4
- 7+4
That's 10/80 = 12.5%
There must be something wrong with your program or...