My solution, it´s make with jquery-chosen and bootstrap, the id is for jquery chosen, tested and working, I had problems concatenating @foreach but now work with a double @foreach and double @if:
<div class="form-group">
<label for="tagLabel">Tags: </label>
<select multiple class="chosen-tag" id="tagLabel" name="tag_id[]" required>
@foreach($tags as $id => $name)
@if (is_array(Request::old('tag_id')))
<option value="{{ $id }}"
@foreach (Request::old('tag_id') as $idold)
@if($idold==$id)
selected
@endif
@endforeach
style="padding:5px;">{{ $name }}</option>
@else
<option value="{{ $id }}" style="padding:5px;">{{ $name }}</option>
@endif
@endforeach
</select>
</div>
this is the code por jquery chosen (the blade.php code doesn´t need this code to work)
$(".chosen-tag").chosen({
placeholder_text_multiple: "Selecciona alguna etiqueta",
no_results_text: "No hay resultados para la busqueda",
search_contains: true,
width: '500px'
});