Regulations mandating consumer consent for data collection and use have led firms to employ "dark patterns"—interface designs that encourage data sharing. We study the causal effects of these designs on consumer consent choices and explore their implications for consumer welfare. We conduct a field experiment where a browser extension randomizes consent interfaces while study participants browse the internet. We find that consumers accept all cookies over half of the time absent dark patterns, with substantial user heterogeneity. Hiding options behind an extra click significantly sways choices toward visible options, while purely visual manipulations have smaller impacts. Users also frequently close banners without active selection, highlighting the importance of default settings. While better-known firms achieve higher baseline consent rates, dark patterns do not amplify this advantage. We use a structural model to show that the consumer surplus-maximizing banner removes dark patterns and defaults to accepting cookies upon consumer inaction. A browser-level global consent choice further improves consumer welfare compared to site-by-site consent interactions, primarily by reducing time costs.