Hi @kessy ,
Have you tried a different charger or connected the phone to a USB port in a computer to see if the battery charges properly?
If so and it is no different perhaps the battery is failing or there is a problem with the phone's charging circuit.
Unfortunately BLU only give a 6 months manufacturer's warranty on the battery (in the USA that is) and as you bought the phone in March, your battery is now out of warranty
So to determine if it is the battery or the phone that is the problem you will have to buy a replacement battery (search online for a supplier that suits you best) and find out.
(I think that if you return the phone to BLU for a warranty repair, suspecting that it may be the phone that is the problem and it turns out to be a defective battery that they will charge you for finding this out.)
If a new battery doesn't solve the problem then most certainly you should make a claim for a manufacturer's warranty repair as BLU warrants the phone for 12 months. Consult the warranty statement that came with the documentation that came with the phone or here is a link to find out what you have to do to make a warranty claim