Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concern.... As per your complain the first thing that I would like to tell you is that in case if you have thrush and you continue to take antibiotics then the bacterial flora f the mouth that is responsible to resist the supernumerary or opportunistic infections like Thrush to proliferate will get disrupted along with the harmful bacteria leading to progression of the infections like Thrush to a more severe level.. As you have thrush and also you are taking antibiotics then the resolution of the fungal infection will be slow or can also be persistent.. You should consult your treating doctor and get evaluated and if possible the doctor should be asked to keep the dose of antibiotics as minimum as possible.. For thrush you have to continue taking antifungal oral medicines, mouthwashes and lozenges.. Add more of probiotics like Yoghurt and garlic in diet.. Maintain a good oral hygeine and drink plenty of water to avoid dehydration as thrush can cause dry mouth.. You should avoid use of antimicrobial mouthwashes and can gargle with warm saline water.. The blue black veins under the tongue [Lingual veins] is not any abnormality and they are blue to black in appearance naturally and as they are not causing you any discomfort you need not to worry about them as there is nothing wrong.. Hope this information helps.. If you find the answer helpful please give me a Five Star Review and click on found the answer helpful as a token of appreciation.. Thanks and regards.. Dr.Honey Nandwani Arora..