1 in 16 mathmatical odds--but Patternless albinos (and Blazing blizzards)aren't that easily made from 'scratch'. Using the Tremper Albino strain, you'd be lucky to hatch 1 out of 100 (or worse) double homozygous for both patternless and Tremper albino when using double hets...some genes are ALOT harder to get to combine than others...
Hey, Steve at Geckos etc has a whole breakdown of two different ways to get a bb or albino patty. It's on his genetics page. Might be helpful. Good luck