There are 2 solutions for it depending on the syntax/content of your file.

1. Specify the character - as delimiter at /Delimiters = and specify - in a color group /Cx"..." which also uses the blue color like the numbers.
   
2. Create a color group for negative numbers and use also the blue color for this color group. The negative numbers are defined with the substring definition. Example:
   
   /C8"Negative Numbers"
   ** -0 -1 -2 -3 -4 -5 -6 -7 -8 -9

Solution 2 has the advantage that words with a - in the middle like text-align are still recognized as one "word". But with solution 2 a delimiter character must exist before every negative number.