Hi ihl2016,
As Justine mentioned, contamination depends on how your samples are being processed. As an illustration, if people deal with different samples/matrices in the same place or in the same laminar flow cabinet, cross-room contamination, well contamination during extraction/library prep.
Last week I've got something close to what your seeing.
My case we didn't run some samples but either didn't exluded them from the SampleSheet. Turns out they have got reads and I could assign taxonomy for them. But I believe it was nothing more than noise.
I believe the number of reads from your blank samples is low and may be nothing, but if your guessing that a likely contamination could be around, give it a try in what Nicholas suggested me in that thread.
Cheers
Leo