first of all,why doesnt after effects renders transitions used in premiere.it only recognises cross disolves and not dip to blacks.how do i go about this.

You can create fades and dissolves using the "opacity" property with keyframes. After effects will read these keyframes correctly.

sorry how do i crossfade with keyframes in adobe premiere

Line up the clips so they overlap on the timeline, and then dissolve the other clip overtop of it from 0-100% with the duration you want from it (default is 1s or ;30 frames).

After effects will pick that up no problem when your switching things back and forth. Had this same problem before and had to adjust how I did things a little bit.