Allston’s Blanchard’s Liquors has apologized to its customers, saying a “cyber attack” compromised customers’ financial information, and said it would know more about the extent of the attack at the end of the week. In a message posted on the store’s website Monday, Blanchard’s said it was working with law enforcement and IT consultants to identify victims whose accounts may have been compromised after using credit and debit cards at the store on the corner of Brighton and Harvard Avenues, just west of Boston University.