What is this part of blastula called and how is endoderm formed?
The blastocoel, as you say yourself, is a fluid-filled cavity (wikipedia) so it has no cells. Fig. 1 shows cellular and non-cellular regions of the blastocyst.

Your questions on the formation of the three germ layers during germination are nicely shown in the illustration in Fig. 2, right two panels.


Fig. 1. Blastocyst. source: wikipedia


Fig. 2. Formation of the germ layers. source: Class Connection