If they withdrew blood from this location, then it would still likely have been a vein, although a smaller vein than the vein that is usually stuck.
Usually, if the needle is inserted into a muscle or other structure near a vein, then no fluid is obtained, or at most, only a small amount of fluid. This small amount of fluid would typically be clear and either colorless or slightly yellow in color.
If the tube filled with blood, then they must have been in some sort of a vascular structure. It would typically be a vein, although sticking an artery can also yield blood. For blood drawn from any vascular structure, the BAC would generally be accurate.
If the needle was partly inside a vein and partly outside a vein, then the blood may have gotten diluted with a small amount of the clear fluid noted above, which may also dilute any lab tests. But the small amount of fluid would usually only make a small difference in the BAC.
If I can provide any additional information, please let me know.