The blue-black color is an indication that your Fe-TA coordinated complex has formed. To avoid precipitation, you need to find an optimum molar ratio of Fe and TA to work with.

Prayasee Baruah thank you, but i found the stoichiometry of the complex M: L (3:1) with three method, molar ratio and slope ratio, continuous variation

Then you can lyophilize the complex and use it for further applications.

Tannic acid contains gallic acid units which has three phenolic hydroxide group. The pKa of the first one is going to be very acidic. The Fe(III) being tri positively charged, it will deprotonate that one even at lower pH. The Fe(III) binding is favoured also because of chelate ring formation and HSAB principle (Fe(III) is hard acid, Oxygen donor is hard base). Overall, catechols (part of gallic acid unit) can bind Fe(III) even at lower pH. This is quite siilar to binding of Fe(III) with enterobactin.