The phrase "sure as blazes" is an idiomatic expression used to convey certainty or confidence in a statement. If you're looking for synonyms to replace it, there are several options available. You can use phrases like "absolutely certain," "completely convinced," "confidently sure," "definitely," "undoubtedly," or "positively certain." Other phrases that can be used as alternatives to "sure as blazes" include "without a doubt," "as sure as the sun rises," "as sure as night follows day," "as certain as death and taxes," and "as predictable as gravity." Whatever phrase you choose, make sure it conveys the exact level of confidence and certainty that you intend to express.