Extended Euclidean algorithm with negative numbers

Well, if you strip the sign of $a$ and $b$, and instead run the Euclidean algorithm for $|a|$ and $|b|$, then if your result is $|a|x+|b|y=1$, you can still get a solution of what you want because $$a(\text{sign}(a)\cdot x)+b(\text{sign}(b)\cdot y)=1.$$

You can just change the signs of $x$ and $y$ appropriately.