Yep, you can only join on dimensions, as far as I know. ID fields should usually be dimensions because you normally won't want/need to aggregate them. When you first connect to your data Tableau takes a look at the first record below the header and makes a guess at whether to make a field a dimension or a measure, which is why IDs get posted as measures (they are numeric). But these data windows are only the defaults, which you can change by dragging fields between the two windows.
One oddity in this dragging between the windows is that while you can NOT drag a date type to the measure pane, you can drag a date to the viz and then convert it to measure CNT(Shipping Date) for instance. Why? Who knows. Just one of those T inconsistencies to remember.
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You've got the correct approach. Data Blending is done across common dimensions. From your explanation here, I believe that ID is a dimension field anyway. Tableau is placing this field in the measures pane because of the data type. As you have rightly pointed, change the measure to a dimension (drag and drop) and then use it for the blend!
Happy Blending! :-)