The Halloween spike in candy sales should be especially good for chocolate products, according to Rockville-based market research firm Packaged Facts. Last year, sales of seasonal Halloween chocolate candy reached $217 million, up 12 percent. That growth outperformed purchases of seasonal chocolate products during Christmas and matched percentage gains for Valentine’s Day. Hershey and Mars together account for roughly 87 percent of Halloween chocolate sales. The overall U.S. market for chocolate grew 4 percent last year, reaching $21 billion.