If you have blue in the areas shadowed by the leaves, it indicates something exposed or fogged the sensitizer in those areas. It's certainly conceivable that some chemical in the celery might have done this, but much more likely a problem with impurities in the paper, contamination (as by a metal ferrule on a brush used to spread sensitizer), or some impurity in drops of water left on the leaves (if you washed them before using them to shadow the cyanotype). If you were using a peroxide rinse to deepen the color more quickly, splashed drops of peroxide solution before washing off the unexposed sensitizer could also produce this effect.