If you used a 4 sided boundary blend then it is one surface. The lines within the boundary are likely the result of changes in the surface properties (isoparms). Not a definitive diagnosis from your picture only, would need to look at the model to be sure.
In order to not get the lines within the boundary you will need to "clean" up your boundary curves.
Could you please provide part file with the curves you used to create that surface? Curve approximation may be one way but it may give you geom check error in the subsequent surface merges. I would like to look at your curves and try it out. Thanks