What is large blue oblong pill - with no imprint?

Actually the pill is an antibiotic for k-9,much akin to penicillin 500mg. That is if it has lines to break it in half or thirds. 1 pill is equal to a full days dosage usually. Assuming a child took one per say it would only be of serious /major concern if said individual were in infancy and/or allergic to the 'cillin medications. But always your BEST bet if it wasn't prescribed flush it. Simple as that. There is always a danger involved when taking medication which your physician hasn't prescribed to you directly, as each case and need are unique unto themselves.
I do hope this was of help to you even after all this time

It is impossible to ID any pill without an imprint - sorry.

All Rx and OTC drugs in the US are required by the FDA to have an imprint.

Your pill could be one of the following – diet, energy, herbal, vitamin, illicit or foreign.
I hope this helps you!