Data-blending is a way of using different fields from different Data Sources in one Worksheet. There is always a primary data source (the tick icon in blue) and one or multiple secondary data sources (the tick mark in orange). As for joins, one or multiple common fields are needed to create the relationships between the data sources.
Unlike joins, data-blending is often used to add new Measures. Fields coming from the secondary data sources are always aggregated. However, they are only aggregated using the common fields, so there is no data duplication. Finally, data-blending can rapidly have a negative impact on performance.